Review ArticleModulators for Terahertz CommunicationThe Current State of the Art

Z T Ma1 Z X Geng23⋆ Z Y Fan3 J Liu4⋆ and H D Chen3⋆

1College of Science Minzu University of China Beijing 100081 China2School of Information Engineering Minzu University of China Beijing 100081 China3State Key Laboratory for Integrated Optoelectronics Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 China4State Key Laboratory of Superlattices and Microstructures Institute of Semiconductors Chinese Academy of SciencesBeijing 100083 China

⋆Correspondence should be addressed to Z X Geng zxgengsemiaccn J Liu liujiansemiaccnand H D Chen hdchensemiaccn

Received 5 November 2018 Accepted 18 March 2019 Published 29 May 2019

Copyright copy 2019 Z TMa et al Exclusive Licensee Science andTechnologyReviewPublishingHouseDistributed under aCreativeCommons Attribution License (CC BY 40)

With the increase of communication frequency terahertz (THz) communication technology has been an important researchfield particularly the terahertz modulator is becoming one of the core devices in THz communication system The modulationperformance of a THz communication system depends on the characterization of THz modulator THz modulators based ondifferent principles and materials have been studied and developed However they are still on the way to practical applicationdue to low modulation speed narrow bandwidth and insufficient modulation depth Therefore we review the research progressof THz modulator in recent years and evaluate devices critically and comprehensively We focus on the working principles suchas electric optical optoelectrical thermal magnetic programmable metamaterials and nonlinear modulation methods for THzwave with semiconductors metamaterials and 2D materials (such as graphene molybdenum disulfide and tungsten disulfide)Furthermore we propose a guiding rule to select appropriate materials and modulation methods for specific applications in THzcommunication

1 Introduction

The terahertz (THz) wave is firstly used to describe thespectral line frequency coverage of Michelson interferometerby Fleming in 1974 [1] It is referred to as the ldquoTHz gaprdquoin the electromagnetic spectrum until the mid-1980s [2 3]due to the lack of effective methods for generating anddetecting THz radiation With development of femtosecondlaser the technology of THz wave generation and detectionhas been improved [4] Generally THz wave is referred to asthe frequency range from 01THz to 10THz (correspondingto wavelengths between 30 120583m and 3 mm) which is thefield of the transition from electronics to photonics [56] THz wave position in the electromagnetic spectrum isshown in Figure 1 THz wave occupies a crucial frequencyrange because it usually carries vital physical information [7]which can be used in cutting-edge technologies [8 9] suchas wireless high-speed communications [10 11] biomedicaldiagnostics [12ndash14] security imaging [15 16] signal detection[17] and product quality control [18 19] However the

existing optical and microwave theories are unsuitable forTHz wave perfectly [8]

In particular THz wave is responsible for communi-cations to achieve 10 Gbps wireless transmission speedsthat are 100 to 1000 times faster than the current ultra-wideband (UWB) technologies Naturally THz band asunoccupied spectrum resources is new choice for com-munication THz communication has the merits whichcombined that of microwave communication and opticalcommunication [20ndash22] (1)The frequency of THz wave is1sim4 orders higher than microwave communication whichexhibits a larger communication capacity (2) The narrowbeam and good directivity of THz wave lead to strong anti-interference ability and high security (3) THz wavelengthis relatively short Therefore the size of antenna is smallwhich makes THz communication system relatively simpleand compact THz communication has maybe grown upas next generation communication technology [23ndash26] Toclarify the differences among THz wave microwave andvisible-infrared light in communication the comparison of

AAASResearchVolume 2019 Article ID 6482975 22 pageshttpsdoiorg103413320196482975

2 Research

Frequency(THz)

Wavelength(nm)

Microwaves IR UV

Electronics PhotonicsTHz

10minus3 10minus2 10minus1 1 10 102 103 104

3times108 3times107 3times106 3times105 3times104 3times103 3times102 30

Radiowaves Vis X-Rays

Figure 1 THz wave position in the electromagnetic spectrum

Table 1 The comparison of different carrier communication characteristics

Carrier Microwave THz Opticalcommunication communication communication

Transmission mode Wireless networks Wireless networks Wired networksTransmission distance Long range Visual range Ultra-long rangeMessage capacity Mbits Between GbitsDirectionality Bad Between FineSecurity Low Low-radiation High photon energy

different carrier communication characteristics is presentedin Table 1

As seen from Table 1 it illustrates that THz wave tech-nology is equivalently suitable for communication occasionswith various special requirements such as close-range securecommunication and space-based communication [27 28]In THz communication system modulators play a key roleConsequently it has become a ldquohotrdquo research field More andmore materials have been used to develop THz modulationdevices currently such as photonic crystals metamaterialstructures phase change materials high electron mobilitytransistors (HEMTs) structures and graphene

After reviewing and comparing recently developed THzmodulators we presented merits and drawbacks of differentTHz modulators These THz modulators were classified byvarious approaches such as electronic optical photoelec-tric thermal magnetic modulation The materials used inmodulators focus on semiconductors two-dimensional (2D)materials and metamaterials At last the future developmentdirection and application of THz modulator was proposed

2 THz Modulation Technique

The THz communication mainly depends on the THz mod-ulation and demodulation technology THz detection andreception technology and THz generation technology Therational use of modulators can effectively reduce the com-plexity cost and geometry of THz systems Consequentlymodulation technology is the focus of research in THzcommunication technology Signal modulation refers to a

process of using the modulation signal to control one ormoreparameters (amplitude phase etc) of the carrier signal [29]In recent years various THz modulators based on differentmaterials and structures [30ndash32] have been reported toachieve large modulation depth fast modulation speed andwide modulation bandwidth [33] Figure 2 shows a diagramof the elementary THz communication [9]

21 Optically Tuned THzModulator The earliest THzmodu-lator is based on hybrid multiquantum well structure whichis optically tuned to realize THz modulation [34] Opticalmodulation often used a laser as the excitation light sourceThe laser irradiates the substrate material to generate carrierswhich can affect the conductivity of the material The changein conductivity changes the transmittance and reflectivity ofthe THz wave transmitting in the material which realizes themodulation of THz wave [35 36]

211 Modulator Based on Semiconductor Materials andMeta-materials The semiconductor materials and metamaterialscan artificially control the electromagnetic waves by design-ing the reasonable structure and size of unit cell [37] In 2004Tanaka et al used 2D metal hole arrays (2D-MHAs) as aTHz modulator [38] Inspired by this work many researchgroups began to use laser to modulate the THz wave InTHz wave region the electric field response mechanism ofconventional semiconductor materials can be explained bythe Drude model [39] which treats free carriers in semicon-ductor materials as free electron gas When laser irradiateson semiconductor materials free carriers may be generated

Research 3

Terahertz transmissionTerahertz resource and modulation Terahertz transmission acception

100GHz fiber Tx module T to E Rx moduleSignal processing

Figure 2 Diagram of THz communication

so long as the energy of the photon ℎ120596 is greater than thebandgap of the semiconductor 119864119892 The equivalent relativedielectric constant is given as

120576119890119891119891 (120596) = 1 minus 12059621199011205962 + 119895Γ120596 (1)

The plasma frequency is given as

120596119901 = radic 11987311989021205760119898119890119891119891 (2)

where Γ is related to the propagation loss 120596119901 is the electronplasma frequency119873 is the electron density 119890 is the electroncharge119898119890119891119891 is the effective mass of the electron

For semiconductor materials at normal temperaturemicrowave or THz wave transmitting in the material will bestrongly attenuated and it cannot even propagate For theultraviolet or higher band of 120596 ge 120596119901 materials are almostldquotransparentrdquo To obtain an electric field response in THzwave it is necessary to lower the electron plasma frequency120596119901 of materials From the expression of plasma frequency itis well known that the electron plasma frequency of materialscan be varied by changing the effective mass and density ofthe electron Therefore using a laser with an energy greaterthan the forbidden bandwidth of materials can modulate theTHz wave

A THz modulator by exciting free electrons in a hybridmultiple-quantum-well structure was demonstrated by Libonet al The electron density could reach 1011 cmminus2 in a periodicquantum well structure It realized a modulation depth of40 in the range of 02-1 THz [34] This was the firstdemonstration of optically tuned THz wave In subsequentfollowing research semiconductor materials commonly usedinclude high-resistance Si high-resistance GaAs and Si onsapphire Meanwhile combined with some structural designsuch as photonic crystal anisotropic medium and surfaceplasma array modulator could achieve better modulationeffect The modulator used in Lirsquos work was a high-resistanceSi wafer [40] Unfortunately the modulation speed was only02 kHz given that the lifetime of carriers in ultra-high-resistivity Si wafer was long On the contrary the lifetime ofcarriers in GaAs is short Fekete et al reported a method ofembedding a GaAs defect layer in alternately stacked SiO2andMgOperiodic structures to constitute a one-dimensional

photonic crystal The efficient modulation of the THz beamcan be achieved even at low photocarrier concentrations byexciting the front GaAs surface via ultrashort 810 nm laserpulses [41]

Theoretically the modulation speed of the THz wavecan reach to GHz The difficulty lies in the carrier lifetimeof the semiconductor substrate material Usually high laserenergy can also solve this problem The pump laser dynam-ically modifies the plasma frequency of the localized surfaceplasmon rather than changing conductivity In 2013 Denget al experimentally demonstrated the modulation speedof 12 GHz on InSb gratings fabricated on semi-insulatingGaAs substrate (Figure 3(1)) This provided more creativepossibilities for optical tuning in future THz devices [42]Modulation efficiency depends on the energy level of thematerial the crystal arrangement of the organic moleculesand the transported carriers at the interface Actually theactive modulators can implement near-perfect modulationefficiency for THz communication applications [43] Yoo etal used a hybrid dual-layer system consisting of molecu-lar organic semiconductors and Si to implement opticallycontrolled active THz modulator (Figure 3(2)) The 98modulation efficiency was achieved due to the rapid light-induced electron transfer from Si to C60 layer which isalmost fully modulated At present some studies have shownthat the deposition of organic film on Si can also achievefull modulation [45 46] Since nanomaterials have uniquephysical properties semiconductor nanostructures become aresearch hotspot [47] The modulator used in Shirsquos work wasa Si nanotip (SiNT) array (Figure 3(3)) The nanotip couldbe used as a THz wave antireflection layer to achieve lowloss This optically driven THz modulator with low loss andhigh modulation depth has the potential to be used in THzimaging [44]

The negative refractive index metamaterial is mainlyrealized by lattice of thin wires and open resonant ringsThe unique electric field response of lattice of thin wires canachieve a negative dielectric constant accordingly the mag-netic field response of the open resonant rings can achievea negative magnetic permeability [48 49] Like the THzmetamaterial modulator reported by Padilla the modulatorconsists of a metal open resonant ring array structure ona GaAs substrate When laser is illustrated on the GaAssubstrate photogenerated carriers affect opening capacitancewith varying laser power Thus the transmission intensityof the THz wave would be modulated [50] ldquoMetasurfacesrdquo

4 Research

1

2

3

(a)

(a)

(a)

(b)

(b)

(b)

400

350

300

250

200

150

100

50

THz A

mpl

itude

(mV

)

THz waves

CuPc pentacene

organic material100 sim 5000 nm

Si substrate500 m

Laser Power (mW) Laser Power (mW)0 200 400 600 800 1000

Delay time (ps)

fitting

100 200 300 400 500 600

0 200 400 600 800 1000

10

08

06

04

02

00

10

08

06

04

02

Mod

ulat

ion

Dep

th

Nor

mal

ized

TH

z sig

nal (

au)

Bare Si25um

50um10um

Bare Si25um

50um10um

293 JcＧ2

147 JcＧ2

74 JcＧ2

23 JcＧ2

074 JcＧ2

60

600050004000300020001000

0minus1000minus2000minus3000minus4000minus5000minus6000

Die

lect

ric co

nsta

nts

04 06 08 10 12 14 16

Frequency (THz)

04 06 08 10 12 14 16

Frequency (THz)

1000

800

600

400

200

0

Real

Imaginary

Con

duct

ivity

(Ω-1cm

-1)

SiRT80 ∘C100 ∘C

120 ∘C140 ∘C160 ∘C

Figure 3Optically tunedTHzmodulator based on semiconductormaterials andmetamaterials Panel (1) (a) SEM images of the fabricated InSbgrating (b) Normalized THz signals as a function of delay time measured by OPTP under different pump laser fluences and the exponentialcurve fittings Reproduced from [42] Panel (2) (a) Schematic view of THzwave transmissionmeasurement for bilayer samples (b) Dielectricfunctions obtained from pentaceneSi thermally annealed at various temperatures Reproduced from [43] Panel (3) (a) Prototype and spatialconfiguration of the Si-nanotip-based spatial THz modulator (b) THz transmission amplitude (left) and the modulation depth (right) ofdifferent modulators as a function of the laser pumping power Reproduced from [44]

are the two-dimensional version ofmetamaterials Comparedto three-dimensional bulk metamaterials the thickness ofthe metasurfaces relative to the operating wavelength isnegligible They consume less physical space and have lowerinsertion loss Longqing Cong et al demonstrated an active

hybrid metasurface integrated with patterned semiconductorinclusions for all-optical active control of terahertzwaves [51]It achieved ultrafast modulation of the polarization state Byproperly incorporating silicon islands into the metamaterialunit cells Xieyu Chen et al presented a metasurface which

Research 5

Valence band

Dirac point

Conduction band

EHole

Electron

Fermi level Fermi level

Fermi level

(a) (b) (c) (d)

＋Ｓ

＋Ｒ

Figure 4 Schematic diagram of energy band structure of graphene (a) Band structure of graphene (b) Fermi level is at Dirac point (c) Fermilevel is in valence band (d) Fermi level is in conduction band

can be optically controlled with amodulation depth reaching68 and 62 for horizontal and vertical polarizations [52] Itopens up new avenues for the design of active metamaterials

212 Modulator Based on 2D Materials 2D materials haveunusual electrical and optical properties In recent years theyattracted increasing attention for applications in optoelec-tronics Graphene the best-known 2D material has beenwidely used for THz modulators tuned by light since beingdiscovered in 2004 [53] The carbon atom arrangementof graphene determines its unique conical band structureThe conductivity of graphene is contributed by the in-bandtransition of electrons and the transition between bandsIn the THz range the in-band transition of electrons playsa decisive factor due to the photon energy being smallTherefore we can approximate the thin layer conductivity ofgraphene by the Drude model as [54]

120590 (120596) = minus 119895119863120587 (120596 minus 119894Γ) (3)

119863 is the Drude weight and it is given as

119863 = V1198651198902ℏ (4)

V119865 = 119864119865ℏradic|119899| (5)

where Γ contributed to the acoustic phonon scattering V119865 isthe Fermi velocity 119899 is the carrier concentration

Thus the thin layer conductivity of graphene is closelyrelated to the Fermi level In general the concentration andtype of carriers can be dynamically modulated by changingthe Fermi level position of graphene Figure 4(a) is the bandstructure of grapheme [55 56] When the Fermi level is inthe conduction band the main carrier is free electrons whenthe Fermi level is in the valence band the main carrier isa hole when the Fermi level is at Dirac point the carrierconcentration is at a minimum and the conductivity ofgraphene is also very low

Graphene has low insertion losses which is extremelyideal for optical modulation Zhang Xiangrsquos team firstly

demonstrated it They proposed an optical modulator by reg-ulating the Fermi level of graphene [57] The most graphenemodulators consist of graphene and semiconductor materialto formheterojunction Laser beampenetrates Si to produce alarge number of carriers meanwhile photogenerated carriersdiffuse into the graphene layer and change its conductivitywhich results in an amount of THz wave absorption bygraphene layer The most classic work is light-modulatinggraphene devices formed with graphene on Si (GOS) demon-strated by Weis in 2012 (Figure 5(1)) This modulator waspumped by a 780 nm femtosecond laser The graphene layerabsorbed the modulated beam of approximately 23 Asexcited with wavelength of 750 nm and power of 40 mWpumped laser this modulator could realize the modulationabout 68 The tunable THz bandwidth was in the range of02-2 THz When the optical pump energy reached 500 mW(750 nm) the transmission THz wave almost completelydisappeared [58] By replacing Si with Ge an optical modula-tor of full modulation based on graphene was demonstrated(Figure 5(3)) A laser with a wavelength of 1550 nm could beused to pump effectively The measurement results presentedthat the modulation frequency range modulation depthand modulation speed were 025-1 THz 94 and 200kHz respectively [59] Although it achieved full modulationit required a high-power laser With development of 2Dmaterial fabrication an efficient THz modulator with lowpower was demonstrated It needs only an external 450 nmcontinuous wave laser It is mainly coated with a high-qualitymonolayer graphene filmon a Si substrateWith external lightexcitation the proposedmodulator could reach amodulationdepth of 74 Incidentally its modulation depth can befurther improved [60]

Monolayer transition metal dichalcogenides (TMDssuch as MoS2 and WS2) are direct-bandgap semiconductorsoffering properties complementary to graphene [89] A greatnumber of modulators based onmonolayer TMDs have beenreported For example a bidimensional material modulator(Figure 5(4)) based on MoS2 and Si improved modulationdepth as high as 649 at 09 THz pumped with an 808 nmlaser The modulation can be much higher after annealingMoS2 Under a larger pump power of 456 W this depth wasabout 96 These results suggested that MoS2 is a promising

6 Research

1

2 3 4

5 6

(a)

pow

er fl

ow graphene

losslesssilicon

attenuatedTHz-beam

infrared-beam

distance

THz Wave Si layer

THz wave

pump light

Detector

-299 nm

355 nm

Reflection

Reflection

CW Laser

CW Laser

Transmission

Transmission

OrganicSi

OrganicSi

THz

THz

THz Camera

THz-CWs

Si

Modulator

Si

808nmLaser

1550nm

Laser

EH

EH

siliconconducting

(b)

(b)(a) (c)

W３2

W３2 layer

10

08

06

04

02

00

0706

05

04

03

02

01

00

Tran

smiss

ion

07

06

05

04

03

02

01

Tran

smiss

ion

0 100 200 300 400 500

Modulation beam power (mW)0 100 200 300 400 500

Modulation beam power (mW)

Modulation beam power (mW)0 10 20 30 40 50

Modulation beam power (mW)0 10 20 30 40 50

SiliconGOSModel

Mod

ulat

ion

dept

h

08

06

04

02

00

Mod

ulat

ion

dept

h

SiliconGOSModel

Figure 5 Optically tuned THz modulator based on 2D materials Panel (1) (a) Schematic view of graphene on Si sample (b) Normalizedtransmission (left) and depth (right) from the three phthalocyanine structures Reproduced from [58] Panel (2) Layer structure of the THzmodulator based on WS2 and Si Reproduced from [61] Panel (3) The modulator consists of a single-layer graphene sheet on a germaniumsubstrate The beam of the THz wave is completely overlapped by the laser beam Reproduced from [59] Panel (4) A sketch map of theexperiment Reproduced from [62] Panel (5) Experimental setup of the THz-CW for measuring transmission and reflection Reproducedfrom [45] Panel (6) (a) The AFM image of liquid-exfoliated WS2 nanosheets (b) The image of the prepared WS2-Si sample (c) Thicknessdistribution map of WS2 film measured by white light interferometer Reproduced from [63]

material [62] Later on many studies have been reported forimprovement in terms of cost manufacturing process pumppower response speed and modulation depth Liu et alstudied a highly efficient activeTHzwavemodulator based onMoS2Ge structure Monolayers of MoS2 and graphene sam-ples were grown on n-doped Ge substrates by chemical vapordeposition (CVD) The light control bandwidth was greatlywidened to reach 26 THz [90] In addition to the expansionof the modulation bandwidth Fan et al demonstrated a THzmodulator (Figure 5(2)) based on p-type annealed tungstendisulfide (WS2) and high-resistivity Si (n-type) structureThismodulator presented a laser power-dependent modulationmechanism Ranging from 025 to 2 THz the modulationdepth reached 99when the pumping laser was 259Wcm2

[61] Other researchers have adopted new methods to realizesimilar modulators The size and thickness of WS2 film arecontrolled by innovatively depositing liquid-released WS2nanosheets on Si instead of CVDmethod (Figure 5(6)) Withraising the pump power the modulation depth continues toincrease eventually reaching 948 under 470mW [63]

213 Modulator Based on Flexible Substrate Compared torigid substrate THz modulators [91] flexible substrates havemany advantages such as transparency lightweight low costand consistent adhesion [9 92] which offer a new potentialfor THz modulator For example in 2006 D Y Khanet al proposed a THz modulation on flexible substrates(Figure 6(1)) They made single-crystal Si with a thickness

Research 7

(a)

(a)

A

Au

Polyimide

Polyimide

ESRRGaAs

GaAs

(b)

(b)

1

2

Bond elements to prestrainedelastomeric substrate

Fabricate thin ribbonSi device elements

stretchableSi devices

Si

PDMS

motherwafer

L+dL

L

Peel back elastomer flip over

A-

09

08

07

06

05

04

03

02

Tran

smiss

ion

04 06 08 10 12 14 16 18

Frequency (THz)

Pump Power0mW025mW05mW1mW2mW

3mW4mW6mW8mW

E

Figure 6 Optically tuned THz modulator based on flexible substrate Panel (1) Schematic illustration of the process for building stretchablesingle-crystal Si devices on elastomeric substrates Reproduced from [64] Panel (2) (a) Schematic of the modulator structure (b) Refractiveindex of the ferrofluid at different magnetic field intensities Reproduced from [65]

of 20 nm to 320 nm into a Si ribbon with a width of 5120583m to 50 120583m and a length of 15 mm When supported bya flexible substrate this wavy Si ribbon could be reversiblystretched or compressed under high horizontal stress withoutdamaging it Considering the electrical properties of Siribbon its electrical parameters can bemodified by changingthe surface shape of the flexible substrate [64] An opticallytuned metamaterial modulator on a flexible polymer sheetwhich had a frequency modulated in the THz range wasproposed by Liu et al Electric split-ring resonators (eSRRs)are attached to a thin polyimide layer and assembled into themodulator (Figure 6(2)) The optical excitation of the GaAspatch changed the response of the metamaterial The meta-material effectively adjusted the effective dielectric constantIn their experiment amodulation depth of 60was achievedin the frequency range of 11-18 THz This kind of flexible

device can be more widely used and created on nonplanarstructures for extensive applications in the future [65] Forcurrent studies high repeatability has not been obtained aftermultiple bending of the flexible substrate due tometal fatigueproperty [78 93]

214 Summary The modulation properties based on dif-ferent materials are summarized in Table 2 The materialdominates the modulation depth and speed of THz waverather than the light source Graphene and other 2Dmaterialscan achieve close to 100modulation depth withmodulationspeed reaching the order of MHz On the other hand moreexperimental works should be carried out to explore thosemodulators based on simulations The modulation efficiencyof optically tuned THz modulator based on flexible substratehas yet to be improved Lower insertion loss and more

8 Research

Table 2 The comparison of optically tuned modulation properties based on different materials

Work Description Year Frequency Modulation depth RefLibon et al Multiple quantum well 1999 02-1 THz 40 [34]Deng L et al InSb gratings 2013 15 THz 4670 [42]Peter W et al Graphene on Si (GOS) 2012 02-2 THz 99 [56]Wen Q Y et al Graphene on Ge 2014 025-1 THz 94 [57]Cao Y et al MoS2 2016 05- 15THz 96 [62]Fan ZY et al WS2 2017 025- 2 THz 99 [61]Hyung K Yet al C60 2014 05- 15THz 98 [43]He T et al AlClPc 2015 0-25 THz 99 [45]Shi Z et al Si Nanotip 2017 025-1 THz gt90 [44]

compact integration will be the future direction of opticallytuned THz modulator

22 Electrically Tuned THz Modulator The other route formodulating THz wave is electrically tuned THz modulatorUsing a mixed type-Itype-II GaAsAlAs multiple-quantum-well sample Libon et al have demonstrated firstly anelectrically controllable modulator in 2000 [94] Electricalmodulation is to control the concentration of electrons inthe substrate materials or structures by applying bias voltagewhich modulate the amplitude of the incident THz wave[94]The transmission bandwidth andmodulation depth andspeed are limited by dielectric constant loss and responsetime of the materials Consequently to achieve high-speedband-pass modulation or broadband filter it is necessaryto find high-speed low-loss THz functional materials anddesign novel structures

Similar to the main idea of optically tuned THz mod-ulator charge injection can also modulate the THz waveIn the past decade 2D electron gas (2DEG) in HEMTshas been demonstrated to modulate THz wave effectivelyHEMT is a field effect transistor that provides a 2DEGquantum well at the heterojunction interface of a highlydoped semiconductor (typically AlGaAs) and an originalundoped semiconductor (GaAs) In 2000 Kersting presenteda GaAsAlGaAs heterostructure device which powered upthe phase modulation of THz signals [94] This modulatorcontained five parabolic quantum wells (PQWs) THz wavemodulation can be realized by stimulating electrically low-energy electron to high-energy states Figure 7(1)(b) showsthe power spectrum of the differential modulation signaland the power spectrum of the incident pulse It led to thedevelopment of THz electronics chip due to a reduction ofthe device dimensions Nevertheless it cannot operate at lowtemperatures To allow THz modulator to be operated atroom temperature Kleine-Ostmann et al presented room-temperature THz wave modulator by using a device based ona gated 2DEG [66] This modulator mainly included a 2DEGwhose density could be controlled by the gate voltage Thismajor breakthrough illustrated that 2DEG in semiconductorscould be used to control THz wave effectively [95] Similarlyit has also been used for THz emission [96] THz detection[97] etc Other 2DEG systems can be also used to realizethe modulation for example a THz modulator based on

2DEGof a GaNAlGaN heterostructure was demonstrated byZhou (Figure 7(3))Themodulator had amaximum intensitymodulation depth of 93 and a 3dB operating bandwidth of400 kHz It required only a low driving voltage amplitude of2V under 87 K This active plasma-based THz modulatormay provide a promising solution in THz technology fieldsfor the metamaterial THz modulator [67]

221Modulator Based on Semiconductors andMetamaterialsIn addition metamaterials are also a hot spot in THzmodula-tor [98] Kebin Fan et al demonstrated a metamaterial activedevice which consists of gold split-ring resonators (SRRs) onGaAs thin film grown on Si substrate [8] It has achieved50 modulation depth and 100 kHz modulation speedSchottkyn-doped GaAs devices had been developed toincrease greatly the modulation speed In further studies themodulation speed had reached 10 MHz however the modu-lation depth of this modulator was only 30 [99]The single-particle nonresonant absorption mechanism described inDrude model has been used in the related studies As seenfrom experiments mentioned above Drude model has notyet been effective enough for the theory of metamaterialmodulator [100]

222 Modulator Based on Graphene To further improve themodulation depth and modulation speed of THz modulatordifferent active medium as substrate has been tested Therefractive index of those materials can usually be written inthe form of a complex refractive index 119899 = 119899119903 + 119894119899119894 where 119899119903is the real part of the refractive index and 119899119894 the imaginarypart of the refractive index When the voltage is adjustedthe real and imaginary parts are usually changed at the sametime By using those changes different electric modulatorscan be developed For example electroabsorption modulatoris based on imaginary part changing When the real partof the refractive index changed with applied voltage thetransmission phase of THz wave changed Therefore forthe electroabsorption modulator based on 2D material abetter modulation effect can be obtained by changing therefractive index through applied voltage In fact electromod-ulators based on 2D materials mainly focused on grapheneGraphene has many particular properties such as adjustablethin layer conductivity and long mean free path due to itsunique conical energy band structure [101ndash103] Its linear

Research 9

1

(a)

THz wave

THz wave

undoped

undoped

SI GaAs

GaAs

GaAsSi

Cr 5 nmCap 10 nmDoped Layer 50 nm

Spacer 7 nm2 DEG

Buffer 1m

Substrate 500 m

2

3

(a)

(a)

(b)

(b)

0

minus4

minus8

minus12

minus16

minus20

minus24

mod

ulat

ion

(pA

)

(a)

(b)

(b)

0 5 10 15 20

time t (ps)

+50 V+20 V+10 V-05 V-10 V-20 V-50 V-100 V

15

10

5

0

5 0 minus5

minus4 minus3 minus2 minus1 0

minus10gate voltage (V)

Gate voltage (V)

peak

-to-p

eak

ampl

itude

(pA

)

D

W

G A

Split gap

E

H

E

H

10

08

06

04

02

00

06

04

05

02

03

01

00

Tran

smiss

ion

Tran

smitt

ance

05 10 15 20

Frequency (THz)0 V1 V2 V4 V8 V16 V

VG

DS

2DEG Sapphire substrate

SI-GaAs

87K20K50K77K

100K120K150K170K

200K230K300K

Ｈ = 14times1018 ＝Ｇ-3

Figure 7 Electrically tuned THz modulator based on semiconductors and metamaterials Panel (1) (a) The structure of the THz modulator(b) Gate voltage dependence of the modulation signal (upper) Peak-to-peak amplitude of the modulator signal vs gate voltage (nether)Reproduced from [66] Panel (2) (a) Geometry and dimensions of the THz metamaterial switchmodulator (b) Frequency-dependenttransmitted intensity of THz radiation Reproduced from [8] Panel (3) (a) Three-dimensional schematic illustration of the plasmon-basedTHz modulator (b) Dependence of transmission on gate voltage at different temperatures Reproduced from [67]

10 Research

(a) (b)

(a)

(c)

(b)

1

2

3

(b)

(a)

Vg

Terahertzbeam

Terahertzbeam

ℎ]

Vg1 Vg2

EF2gtEF1 (Vg2gtVg1)EF1 (Vg1)

||2

10

08

06

04

02

00

Pow

er re

flect

ance

570 590 610 630

Frequency (GHz)

graphene

V

V = 0

２ rarr 1

２ rarr 0

６ = 0

top graphene layer bottom graphene layer

qV

10

08

06

04

02 07

06

0500

Tran

smitt

ance

Tran

smitt

ance

560

580

600

620

640

660

570 575 580 585

Frequency (GHz)Vg = 50 VVg = 0 V

Figure 8 Electrically tuned THz modulator based on graphene Panel (1) (a) Schematic of the proof-of-concept graphene THz modulator(b) Schematic of the THz modulator (c) Schematic of the device and electric field distribution when the substrate thickness and plasmonicresonance are matched to an odd multiple of a quarter wavelength of the THz wave Reproduced from [68] Panel (2) (a) Schematic of theTHz modulator (b) Normalized reflectance (RR (VCNP)) Reproduced from [69] Panel (3) (a) Schematic of the device and electric fielddistribution (b) Operation mechanism of the self-gated graphene pair Reproduced from [70]

dispersion relationship between energy and crystal momen-tum makes graphene superior to other semiconductor mate-rials

S Rodriguez utilized graphene to modulate THz wave(Figure 8(1)) The modulation depth and modulation speed

could reach 15 and 20 kHz respectively [68] Meanwhilethe same research group presented a reflective THz modu-lator as shown in Figure 8(2) The silver film on the backof the reflective THz modulator acted as both gate andmirror which improved the modulation depth with 64

Research 11

Table 3 The comparison of electrically tuned modulation properties based on different substrate materials

Work Description Year Frequency Modulation depth RefHuang Y D Plasmonic 2016 - 93 [67]Zhou G VO2 2017 03-10THz gt50 [100]Sensale-R B Graphene 2012 10THz 100 [106]Liang G Graphene 2015 3THz 94-100 [71]Huang Z Monolayer Graphene 2018 1-7THz gt85 [72]Chikhi N Liquid crystals 2018 1-3THz gt90 [107]Kaya E Flexible Substrate 2018 02-15THz 100 [74]

and expanded bandwidth ranging 057-063 THz comparedto their early works However its insertion loss and mod-ulation speed only reach 2 dB and 4 kHz respectively[69] The essence of those works is to change the Fermilevel of graphene by adjusting applied voltage The follow-ing graphene THz modulators with different structures arealso based on this method To improve modulation depthmultiple layers of graphene are a choice [102] As shown inFigure 8(3) a THz electroabsorption modulator based ontwo-layer graphene with periodic micron-belt pattern waspresented The THz wave was vertically illustrated on thesurface of graphene which enhanced the absorption of lightbecause of the plasma effect This modulator could operateat frequencies up to tens of THz and the modulation depthcould even reach 100 [103]

Usually the modulation depth of most modulators basedon monolayer graphene can only reach 80 [69] Howevermonolayer graphene combined with other structures ordevices can improve the modulation depth For exampleG Liang puts monolayer graphene on top of quantumcascade laser (QCL) to demonstrate modulation depth of100 (Figure 9(1)) and modulation speed of 110 MHz [71]The high modulation depth was a consequence of a stronginteraction between the graphene and THz wave whereasthe high modulation speed can be improved by reducingthe dimension of device It has recently been demonstratedthat a THz modulator can realize full modulation with awide modulation bandwidth and fast modulation speedUsing monolayer graphene Huang proposed an adjustablecomplementary ring resonator (Figure 9(2)) It was worthmentioning that in the range of 1-2 THz and 3-7 THz threesignificant resonant peaks can be modulated The maximummodulation depth reaches 988 at 747 THz [72]

223 Modulator Based on Flexible Substrate Flexible sub-strate was used to fabricate optical modulator with manymerits [104 105] as described in Section 213 For electricmodulator flexible substrate plays also an important role Forexample Kocabas et al presented a graphene metamaterialTHz modulator based on a flexible substrate (Figure 10(1))which consists of two large-area graphene electrodes trans-ferred onto THz transparent substrates with ionic liquidelectrolytes between them The bias can change electrostaticdoping of graphene which affects its optical properties Inthis case the maximum modulation depth reaches only 50at 01-14 THz working at a bias of 3V The modulation speed

was slow [73] Virtually thismodulator showed excellent flex-ibility which has not been deformed after being flexed manytimes Modulator based on flexible substrate can also realizea high modulation depth For example E Kaya et al havedemonstrated that polyvinyl chloride (PVC) and polyethy-lene (PE) were selected as flexible substrates (Figure 10(2))A multilayer graphene modulator was fabricated on thoseflexible substrates by chemical vapor deposition This devicecould fully modulate THz wave with a frequency range of02-15 THz at a low voltage of 34V [74] The comparison ofelectrically tuned modulation properties based on differentsubstrate materials is shown in Table 3

224 Summary The modulation properties of differentmaterials reported in literatures are given in Table 2Graphene can achieve modulation depth close to 100 andthe modulation speed in the order of kHz At present thereare few reports on flexible THz modulators since organicfunctional materials and flexible substrates cannot fully becompatible with micronanofabrication processes Based onreviewing the electric modulator it implied that electricmodulation device needs to miniaturize the structure to lowinsertion loss and to improve modulation speed which willbe the future direction

23 Photoelectric Hybrid Tuned THz Modulator Sections 21and 22 described the modulation methods using electricityor light separately If two methods can be combined togetherbetter modulation effects may be obtained The photoelectrichybrid modulation method is a method of synthesizing elec-tric and optical modulation Its basic principle is using pumplight excited carriers in a base semiconductor material In themeantime the motion direction of the carrier is adjusted byan external bias [108] For example an optoelectric hybridmodulator was exhibited by Q Li et al [75] Its structurewas shown in Figure 12 Si substrate was pumped with acontinuous laser of 532 nm to generate a large number ofelectron-hole pairs Under the drive of the concentrationdifference the photoelectrons in the Si diffused into thegraphene until the equilibrium state was established Astructure similar to a PN junction was formed between thetwomaterials When the bias was varied from 0 V to -3 V thepeak modulation of the time-domain signal peak achieved51 at optical pump power with 420 mW When the biaschanged from 0 V to -4 V the modulation amplitude depthbecame 83 Based on this structure Ran Jiang et al used a

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

Research 3

Terahertz transmissionTerahertz resource and modulation Terahertz transmission acception

100GHz fiber Tx module T to E Rx moduleSignal processing

Figure 2 Diagram of THz communication

so long as the energy of the photon ℎ120596 is greater than thebandgap of the semiconductor 119864119892 The equivalent relativedielectric constant is given as

120576119890119891119891 (120596) = 1 minus 12059621199011205962 + 119895Γ120596 (1)

The plasma frequency is given as

120596119901 = radic 11987311989021205760119898119890119891119891 (2)

where Γ is related to the propagation loss 120596119901 is the electronplasma frequency119873 is the electron density 119890 is the electroncharge119898119890119891119891 is the effective mass of the electron

For semiconductor materials at normal temperaturemicrowave or THz wave transmitting in the material will bestrongly attenuated and it cannot even propagate For theultraviolet or higher band of 120596 ge 120596119901 materials are almostldquotransparentrdquo To obtain an electric field response in THzwave it is necessary to lower the electron plasma frequency120596119901 of materials From the expression of plasma frequency itis well known that the electron plasma frequency of materialscan be varied by changing the effective mass and density ofthe electron Therefore using a laser with an energy greaterthan the forbidden bandwidth of materials can modulate theTHz wave

A THz modulator by exciting free electrons in a hybridmultiple-quantum-well structure was demonstrated by Libonet al The electron density could reach 1011 cmminus2 in a periodicquantum well structure It realized a modulation depth of40 in the range of 02-1 THz [34] This was the firstdemonstration of optically tuned THz wave In subsequentfollowing research semiconductor materials commonly usedinclude high-resistance Si high-resistance GaAs and Si onsapphire Meanwhile combined with some structural designsuch as photonic crystal anisotropic medium and surfaceplasma array modulator could achieve better modulationeffect The modulator used in Lirsquos work was a high-resistanceSi wafer [40] Unfortunately the modulation speed was only02 kHz given that the lifetime of carriers in ultra-high-resistivity Si wafer was long On the contrary the lifetime ofcarriers in GaAs is short Fekete et al reported a method ofembedding a GaAs defect layer in alternately stacked SiO2andMgOperiodic structures to constitute a one-dimensional

photonic crystal The efficient modulation of the THz beamcan be achieved even at low photocarrier concentrations byexciting the front GaAs surface via ultrashort 810 nm laserpulses [41]

Theoretically the modulation speed of the THz wavecan reach to GHz The difficulty lies in the carrier lifetimeof the semiconductor substrate material Usually high laserenergy can also solve this problem The pump laser dynam-ically modifies the plasma frequency of the localized surfaceplasmon rather than changing conductivity In 2013 Denget al experimentally demonstrated the modulation speedof 12 GHz on InSb gratings fabricated on semi-insulatingGaAs substrate (Figure 3(1)) This provided more creativepossibilities for optical tuning in future THz devices [42]Modulation efficiency depends on the energy level of thematerial the crystal arrangement of the organic moleculesand the transported carriers at the interface Actually theactive modulators can implement near-perfect modulationefficiency for THz communication applications [43] Yoo etal used a hybrid dual-layer system consisting of molecu-lar organic semiconductors and Si to implement opticallycontrolled active THz modulator (Figure 3(2)) The 98modulation efficiency was achieved due to the rapid light-induced electron transfer from Si to C60 layer which isalmost fully modulated At present some studies have shownthat the deposition of organic film on Si can also achievefull modulation [45 46] Since nanomaterials have uniquephysical properties semiconductor nanostructures become aresearch hotspot [47] The modulator used in Shirsquos work wasa Si nanotip (SiNT) array (Figure 3(3)) The nanotip couldbe used as a THz wave antireflection layer to achieve lowloss This optically driven THz modulator with low loss andhigh modulation depth has the potential to be used in THzimaging [44]

The negative refractive index metamaterial is mainlyrealized by lattice of thin wires and open resonant ringsThe unique electric field response of lattice of thin wires canachieve a negative dielectric constant accordingly the mag-netic field response of the open resonant rings can achievea negative magnetic permeability [48 49] Like the THzmetamaterial modulator reported by Padilla the modulatorconsists of a metal open resonant ring array structure ona GaAs substrate When laser is illustrated on the GaAssubstrate photogenerated carriers affect opening capacitancewith varying laser power Thus the transmission intensityof the THz wave would be modulated [50] ldquoMetasurfacesrdquo

4 Research

1

2

3

(a)

(a)

(a)

(b)

(b)

(b)

400

350

300

250

200

150

100

50

THz A

mpl

itude

(mV

)

THz waves

CuPc pentacene

organic material100 sim 5000 nm

Si substrate500 m

Laser Power (mW) Laser Power (mW)0 200 400 600 800 1000

Delay time (ps)

fitting

100 200 300 400 500 600

0 200 400 600 800 1000

10

08

06

04

02

00

10

08

06

04

02

Mod

ulat

ion

Dep

th

Nor

mal

ized

TH

z sig

nal (

au)

Bare Si25um

50um10um

Bare Si25um

50um10um

293 JcＧ2

147 JcＧ2

74 JcＧ2

23 JcＧ2

074 JcＧ2

60

600050004000300020001000

0minus1000minus2000minus3000minus4000minus5000minus6000

Die

lect

ric co

nsta

nts

04 06 08 10 12 14 16

Frequency (THz)

04 06 08 10 12 14 16

Frequency (THz)

1000

800

600

400

200

0

Real

Imaginary

Con

duct

ivity

(Ω-1cm

-1)

SiRT80 ∘C100 ∘C

120 ∘C140 ∘C160 ∘C

Figure 3Optically tunedTHzmodulator based on semiconductormaterials andmetamaterials Panel (1) (a) SEM images of the fabricated InSbgrating (b) Normalized THz signals as a function of delay time measured by OPTP under different pump laser fluences and the exponentialcurve fittings Reproduced from [42] Panel (2) (a) Schematic view of THzwave transmissionmeasurement for bilayer samples (b) Dielectricfunctions obtained from pentaceneSi thermally annealed at various temperatures Reproduced from [43] Panel (3) (a) Prototype and spatialconfiguration of the Si-nanotip-based spatial THz modulator (b) THz transmission amplitude (left) and the modulation depth (right) ofdifferent modulators as a function of the laser pumping power Reproduced from [44]

are the two-dimensional version ofmetamaterials Comparedto three-dimensional bulk metamaterials the thickness ofthe metasurfaces relative to the operating wavelength isnegligible They consume less physical space and have lowerinsertion loss Longqing Cong et al demonstrated an active

hybrid metasurface integrated with patterned semiconductorinclusions for all-optical active control of terahertzwaves [51]It achieved ultrafast modulation of the polarization state Byproperly incorporating silicon islands into the metamaterialunit cells Xieyu Chen et al presented a metasurface which

Research 5

Valence band

Dirac point

Conduction band

EHole

Electron

Fermi level Fermi level

Fermi level

(a) (b) (c) (d)

＋Ｓ

＋Ｒ

Figure 4 Schematic diagram of energy band structure of graphene (a) Band structure of graphene (b) Fermi level is at Dirac point (c) Fermilevel is in valence band (d) Fermi level is in conduction band

can be optically controlled with amodulation depth reaching68 and 62 for horizontal and vertical polarizations [52] Itopens up new avenues for the design of active metamaterials

212 Modulator Based on 2D Materials 2D materials haveunusual electrical and optical properties In recent years theyattracted increasing attention for applications in optoelec-tronics Graphene the best-known 2D material has beenwidely used for THz modulators tuned by light since beingdiscovered in 2004 [53] The carbon atom arrangementof graphene determines its unique conical band structureThe conductivity of graphene is contributed by the in-bandtransition of electrons and the transition between bandsIn the THz range the in-band transition of electrons playsa decisive factor due to the photon energy being smallTherefore we can approximate the thin layer conductivity ofgraphene by the Drude model as [54]

120590 (120596) = minus 119895119863120587 (120596 minus 119894Γ) (3)

119863 is the Drude weight and it is given as

119863 = V1198651198902ℏ (4)

V119865 = 119864119865ℏradic|119899| (5)

where Γ contributed to the acoustic phonon scattering V119865 isthe Fermi velocity 119899 is the carrier concentration

Thus the thin layer conductivity of graphene is closelyrelated to the Fermi level In general the concentration andtype of carriers can be dynamically modulated by changingthe Fermi level position of graphene Figure 4(a) is the bandstructure of grapheme [55 56] When the Fermi level is inthe conduction band the main carrier is free electrons whenthe Fermi level is in the valence band the main carrier isa hole when the Fermi level is at Dirac point the carrierconcentration is at a minimum and the conductivity ofgraphene is also very low

Graphene has low insertion losses which is extremelyideal for optical modulation Zhang Xiangrsquos team firstly

demonstrated it They proposed an optical modulator by reg-ulating the Fermi level of graphene [57] The most graphenemodulators consist of graphene and semiconductor materialto formheterojunction Laser beampenetrates Si to produce alarge number of carriers meanwhile photogenerated carriersdiffuse into the graphene layer and change its conductivitywhich results in an amount of THz wave absorption bygraphene layer The most classic work is light-modulatinggraphene devices formed with graphene on Si (GOS) demon-strated by Weis in 2012 (Figure 5(1)) This modulator waspumped by a 780 nm femtosecond laser The graphene layerabsorbed the modulated beam of approximately 23 Asexcited with wavelength of 750 nm and power of 40 mWpumped laser this modulator could realize the modulationabout 68 The tunable THz bandwidth was in the range of02-2 THz When the optical pump energy reached 500 mW(750 nm) the transmission THz wave almost completelydisappeared [58] By replacing Si with Ge an optical modula-tor of full modulation based on graphene was demonstrated(Figure 5(3)) A laser with a wavelength of 1550 nm could beused to pump effectively The measurement results presentedthat the modulation frequency range modulation depthand modulation speed were 025-1 THz 94 and 200kHz respectively [59] Although it achieved full modulationit required a high-power laser With development of 2Dmaterial fabrication an efficient THz modulator with lowpower was demonstrated It needs only an external 450 nmcontinuous wave laser It is mainly coated with a high-qualitymonolayer graphene filmon a Si substrateWith external lightexcitation the proposedmodulator could reach amodulationdepth of 74 Incidentally its modulation depth can befurther improved [60]

Monolayer transition metal dichalcogenides (TMDssuch as MoS2 and WS2) are direct-bandgap semiconductorsoffering properties complementary to graphene [89] A greatnumber of modulators based onmonolayer TMDs have beenreported For example a bidimensional material modulator(Figure 5(4)) based on MoS2 and Si improved modulationdepth as high as 649 at 09 THz pumped with an 808 nmlaser The modulation can be much higher after annealingMoS2 Under a larger pump power of 456 W this depth wasabout 96 These results suggested that MoS2 is a promising

6 Research

1

2 3 4

5 6

(a)

pow

er fl

ow graphene

losslesssilicon

attenuatedTHz-beam

infrared-beam

distance

THz Wave Si layer

THz wave

pump light

Detector

-299 nm

355 nm

Reflection

Reflection

CW Laser

CW Laser

Transmission

Transmission

OrganicSi

OrganicSi

THz

THz

THz Camera

THz-CWs

Si

Modulator

Si

808nmLaser

1550nm

Laser

EH

EH

siliconconducting

(b)

(b)(a) (c)

W３2

W３2 layer

10

08

06

04

02

00

0706

05

04

03

02

01

00

Tran

smiss

ion

07

06

05

04

03

02

01

Tran

smiss

ion

0 100 200 300 400 500

Modulation beam power (mW)0 100 200 300 400 500

Modulation beam power (mW)

Modulation beam power (mW)0 10 20 30 40 50

Modulation beam power (mW)0 10 20 30 40 50

SiliconGOSModel

Mod

ulat

ion

dept

h

08

06

04

02

00

Mod

ulat

ion

dept

h

SiliconGOSModel

Figure 5 Optically tuned THz modulator based on 2D materials Panel (1) (a) Schematic view of graphene on Si sample (b) Normalizedtransmission (left) and depth (right) from the three phthalocyanine structures Reproduced from [58] Panel (2) Layer structure of the THzmodulator based on WS2 and Si Reproduced from [61] Panel (3) The modulator consists of a single-layer graphene sheet on a germaniumsubstrate The beam of the THz wave is completely overlapped by the laser beam Reproduced from [59] Panel (4) A sketch map of theexperiment Reproduced from [62] Panel (5) Experimental setup of the THz-CW for measuring transmission and reflection Reproducedfrom [45] Panel (6) (a) The AFM image of liquid-exfoliated WS2 nanosheets (b) The image of the prepared WS2-Si sample (c) Thicknessdistribution map of WS2 film measured by white light interferometer Reproduced from [63]

material [62] Later on many studies have been reported forimprovement in terms of cost manufacturing process pumppower response speed and modulation depth Liu et alstudied a highly efficient activeTHzwavemodulator based onMoS2Ge structure Monolayers of MoS2 and graphene sam-ples were grown on n-doped Ge substrates by chemical vapordeposition (CVD) The light control bandwidth was greatlywidened to reach 26 THz [90] In addition to the expansionof the modulation bandwidth Fan et al demonstrated a THzmodulator (Figure 5(2)) based on p-type annealed tungstendisulfide (WS2) and high-resistivity Si (n-type) structureThismodulator presented a laser power-dependent modulationmechanism Ranging from 025 to 2 THz the modulationdepth reached 99when the pumping laser was 259Wcm2

[61] Other researchers have adopted new methods to realizesimilar modulators The size and thickness of WS2 film arecontrolled by innovatively depositing liquid-released WS2nanosheets on Si instead of CVDmethod (Figure 5(6)) Withraising the pump power the modulation depth continues toincrease eventually reaching 948 under 470mW [63]

213 Modulator Based on Flexible Substrate Compared torigid substrate THz modulators [91] flexible substrates havemany advantages such as transparency lightweight low costand consistent adhesion [9 92] which offer a new potentialfor THz modulator For example in 2006 D Y Khanet al proposed a THz modulation on flexible substrates(Figure 6(1)) They made single-crystal Si with a thickness

Research 7

(a)

(a)

A

Au

Polyimide

Polyimide

ESRRGaAs

GaAs

(b)

(b)

1

2

Bond elements to prestrainedelastomeric substrate

Fabricate thin ribbonSi device elements

stretchableSi devices

Si

PDMS

motherwafer

L+dL

L

Peel back elastomer flip over

A-

09

08

07

06

05

04

03

02

Tran

smiss

ion

04 06 08 10 12 14 16 18

Frequency (THz)

Pump Power0mW025mW05mW1mW2mW

3mW4mW6mW8mW

E

Figure 6 Optically tuned THz modulator based on flexible substrate Panel (1) Schematic illustration of the process for building stretchablesingle-crystal Si devices on elastomeric substrates Reproduced from [64] Panel (2) (a) Schematic of the modulator structure (b) Refractiveindex of the ferrofluid at different magnetic field intensities Reproduced from [65]

of 20 nm to 320 nm into a Si ribbon with a width of 5120583m to 50 120583m and a length of 15 mm When supported bya flexible substrate this wavy Si ribbon could be reversiblystretched or compressed under high horizontal stress withoutdamaging it Considering the electrical properties of Siribbon its electrical parameters can bemodified by changingthe surface shape of the flexible substrate [64] An opticallytuned metamaterial modulator on a flexible polymer sheetwhich had a frequency modulated in the THz range wasproposed by Liu et al Electric split-ring resonators (eSRRs)are attached to a thin polyimide layer and assembled into themodulator (Figure 6(2)) The optical excitation of the GaAspatch changed the response of the metamaterial The meta-material effectively adjusted the effective dielectric constantIn their experiment amodulation depth of 60was achievedin the frequency range of 11-18 THz This kind of flexible

device can be more widely used and created on nonplanarstructures for extensive applications in the future [65] Forcurrent studies high repeatability has not been obtained aftermultiple bending of the flexible substrate due tometal fatigueproperty [78 93]

214 Summary The modulation properties based on dif-ferent materials are summarized in Table 2 The materialdominates the modulation depth and speed of THz waverather than the light source Graphene and other 2Dmaterialscan achieve close to 100modulation depth withmodulationspeed reaching the order of MHz On the other hand moreexperimental works should be carried out to explore thosemodulators based on simulations The modulation efficiencyof optically tuned THz modulator based on flexible substratehas yet to be improved Lower insertion loss and more

8 Research

Table 2 The comparison of optically tuned modulation properties based on different materials

Work Description Year Frequency Modulation depth RefLibon et al Multiple quantum well 1999 02-1 THz 40 [34]Deng L et al InSb gratings 2013 15 THz 4670 [42]Peter W et al Graphene on Si (GOS) 2012 02-2 THz 99 [56]Wen Q Y et al Graphene on Ge 2014 025-1 THz 94 [57]Cao Y et al MoS2 2016 05- 15THz 96 [62]Fan ZY et al WS2 2017 025- 2 THz 99 [61]Hyung K Yet al C60 2014 05- 15THz 98 [43]He T et al AlClPc 2015 0-25 THz 99 [45]Shi Z et al Si Nanotip 2017 025-1 THz gt90 [44]

compact integration will be the future direction of opticallytuned THz modulator

22 Electrically Tuned THz Modulator The other route formodulating THz wave is electrically tuned THz modulatorUsing a mixed type-Itype-II GaAsAlAs multiple-quantum-well sample Libon et al have demonstrated firstly anelectrically controllable modulator in 2000 [94] Electricalmodulation is to control the concentration of electrons inthe substrate materials or structures by applying bias voltagewhich modulate the amplitude of the incident THz wave[94]The transmission bandwidth andmodulation depth andspeed are limited by dielectric constant loss and responsetime of the materials Consequently to achieve high-speedband-pass modulation or broadband filter it is necessaryto find high-speed low-loss THz functional materials anddesign novel structures

Similar to the main idea of optically tuned THz mod-ulator charge injection can also modulate the THz waveIn the past decade 2D electron gas (2DEG) in HEMTshas been demonstrated to modulate THz wave effectivelyHEMT is a field effect transistor that provides a 2DEGquantum well at the heterojunction interface of a highlydoped semiconductor (typically AlGaAs) and an originalundoped semiconductor (GaAs) In 2000 Kersting presenteda GaAsAlGaAs heterostructure device which powered upthe phase modulation of THz signals [94] This modulatorcontained five parabolic quantum wells (PQWs) THz wavemodulation can be realized by stimulating electrically low-energy electron to high-energy states Figure 7(1)(b) showsthe power spectrum of the differential modulation signaland the power spectrum of the incident pulse It led to thedevelopment of THz electronics chip due to a reduction ofthe device dimensions Nevertheless it cannot operate at lowtemperatures To allow THz modulator to be operated atroom temperature Kleine-Ostmann et al presented room-temperature THz wave modulator by using a device based ona gated 2DEG [66] This modulator mainly included a 2DEGwhose density could be controlled by the gate voltage Thismajor breakthrough illustrated that 2DEG in semiconductorscould be used to control THz wave effectively [95] Similarlyit has also been used for THz emission [96] THz detection[97] etc Other 2DEG systems can be also used to realizethe modulation for example a THz modulator based on

2DEGof a GaNAlGaN heterostructure was demonstrated byZhou (Figure 7(3))Themodulator had amaximum intensitymodulation depth of 93 and a 3dB operating bandwidth of400 kHz It required only a low driving voltage amplitude of2V under 87 K This active plasma-based THz modulatormay provide a promising solution in THz technology fieldsfor the metamaterial THz modulator [67]

221Modulator Based on Semiconductors andMetamaterialsIn addition metamaterials are also a hot spot in THzmodula-tor [98] Kebin Fan et al demonstrated a metamaterial activedevice which consists of gold split-ring resonators (SRRs) onGaAs thin film grown on Si substrate [8] It has achieved50 modulation depth and 100 kHz modulation speedSchottkyn-doped GaAs devices had been developed toincrease greatly the modulation speed In further studies themodulation speed had reached 10 MHz however the modu-lation depth of this modulator was only 30 [99]The single-particle nonresonant absorption mechanism described inDrude model has been used in the related studies As seenfrom experiments mentioned above Drude model has notyet been effective enough for the theory of metamaterialmodulator [100]

222 Modulator Based on Graphene To further improve themodulation depth and modulation speed of THz modulatordifferent active medium as substrate has been tested Therefractive index of those materials can usually be written inthe form of a complex refractive index 119899 = 119899119903 + 119894119899119894 where 119899119903is the real part of the refractive index and 119899119894 the imaginarypart of the refractive index When the voltage is adjustedthe real and imaginary parts are usually changed at the sametime By using those changes different electric modulatorscan be developed For example electroabsorption modulatoris based on imaginary part changing When the real partof the refractive index changed with applied voltage thetransmission phase of THz wave changed Therefore forthe electroabsorption modulator based on 2D material abetter modulation effect can be obtained by changing therefractive index through applied voltage In fact electromod-ulators based on 2D materials mainly focused on grapheneGraphene has many particular properties such as adjustablethin layer conductivity and long mean free path due to itsunique conical energy band structure [101ndash103] Its linear

Research 9

1

(a)

THz wave

THz wave

undoped

undoped

SI GaAs

GaAs

GaAsSi

Cr 5 nmCap 10 nmDoped Layer 50 nm

Spacer 7 nm2 DEG

Buffer 1m

Substrate 500 m

2

3

(a)

(a)

(b)

(b)

0

minus4

minus8

minus12

minus16

minus20

minus24

mod

ulat

ion

(pA

)

(a)

(b)

(b)

0 5 10 15 20

time t (ps)

+50 V+20 V+10 V-05 V-10 V-20 V-50 V-100 V

15

10

5

0

5 0 minus5

minus4 minus3 minus2 minus1 0

minus10gate voltage (V)

Gate voltage (V)

peak

-to-p

eak

ampl

itude

(pA

)

D

W

G A

Split gap

E

H

E

H

10

08

06

04

02

00

06

04

05

02

03

01

00

Tran

smiss

ion

Tran

smitt

ance

05 10 15 20

Frequency (THz)0 V1 V2 V4 V8 V16 V

VG

DS

2DEG Sapphire substrate

SI-GaAs

87K20K50K77K

100K120K150K170K

200K230K300K

Ｈ = 14times1018 ＝Ｇ-3

Figure 7 Electrically tuned THz modulator based on semiconductors and metamaterials Panel (1) (a) The structure of the THz modulator(b) Gate voltage dependence of the modulation signal (upper) Peak-to-peak amplitude of the modulator signal vs gate voltage (nether)Reproduced from [66] Panel (2) (a) Geometry and dimensions of the THz metamaterial switchmodulator (b) Frequency-dependenttransmitted intensity of THz radiation Reproduced from [8] Panel (3) (a) Three-dimensional schematic illustration of the plasmon-basedTHz modulator (b) Dependence of transmission on gate voltage at different temperatures Reproduced from [67]

10 Research

(a) (b)

(a)

(c)

(b)

1

2

3

(b)

(a)

Vg

Terahertzbeam

Terahertzbeam

ℎ]

Vg1 Vg2

EF2gtEF1 (Vg2gtVg1)EF1 (Vg1)

||2

10

08

06

04

02

00

Pow

er re

flect

ance

570 590 610 630

Frequency (GHz)

graphene

V

V = 0

２ rarr 1

２ rarr 0

６ = 0

top graphene layer bottom graphene layer

qV

10

08

06

04

02 07

06

0500

Tran

smitt

ance

Tran

smitt

ance

560

580

600

620

640

660

570 575 580 585

Frequency (GHz)Vg = 50 VVg = 0 V

Figure 8 Electrically tuned THz modulator based on graphene Panel (1) (a) Schematic of the proof-of-concept graphene THz modulator(b) Schematic of the THz modulator (c) Schematic of the device and electric field distribution when the substrate thickness and plasmonicresonance are matched to an odd multiple of a quarter wavelength of the THz wave Reproduced from [68] Panel (2) (a) Schematic of theTHz modulator (b) Normalized reflectance (RR (VCNP)) Reproduced from [69] Panel (3) (a) Schematic of the device and electric fielddistribution (b) Operation mechanism of the self-gated graphene pair Reproduced from [70]

dispersion relationship between energy and crystal momen-tum makes graphene superior to other semiconductor mate-rials

S Rodriguez utilized graphene to modulate THz wave(Figure 8(1)) The modulation depth and modulation speed

could reach 15 and 20 kHz respectively [68] Meanwhilethe same research group presented a reflective THz modu-lator as shown in Figure 8(2) The silver film on the backof the reflective THz modulator acted as both gate andmirror which improved the modulation depth with 64

Research 11

Table 3 The comparison of electrically tuned modulation properties based on different substrate materials

Work Description Year Frequency Modulation depth RefHuang Y D Plasmonic 2016 - 93 [67]Zhou G VO2 2017 03-10THz gt50 [100]Sensale-R B Graphene 2012 10THz 100 [106]Liang G Graphene 2015 3THz 94-100 [71]Huang Z Monolayer Graphene 2018 1-7THz gt85 [72]Chikhi N Liquid crystals 2018 1-3THz gt90 [107]Kaya E Flexible Substrate 2018 02-15THz 100 [74]

and expanded bandwidth ranging 057-063 THz comparedto their early works However its insertion loss and mod-ulation speed only reach 2 dB and 4 kHz respectively[69] The essence of those works is to change the Fermilevel of graphene by adjusting applied voltage The follow-ing graphene THz modulators with different structures arealso based on this method To improve modulation depthmultiple layers of graphene are a choice [102] As shown inFigure 8(3) a THz electroabsorption modulator based ontwo-layer graphene with periodic micron-belt pattern waspresented The THz wave was vertically illustrated on thesurface of graphene which enhanced the absorption of lightbecause of the plasma effect This modulator could operateat frequencies up to tens of THz and the modulation depthcould even reach 100 [103]

Usually the modulation depth of most modulators basedon monolayer graphene can only reach 80 [69] Howevermonolayer graphene combined with other structures ordevices can improve the modulation depth For exampleG Liang puts monolayer graphene on top of quantumcascade laser (QCL) to demonstrate modulation depth of100 (Figure 9(1)) and modulation speed of 110 MHz [71]The high modulation depth was a consequence of a stronginteraction between the graphene and THz wave whereasthe high modulation speed can be improved by reducingthe dimension of device It has recently been demonstratedthat a THz modulator can realize full modulation with awide modulation bandwidth and fast modulation speedUsing monolayer graphene Huang proposed an adjustablecomplementary ring resonator (Figure 9(2)) It was worthmentioning that in the range of 1-2 THz and 3-7 THz threesignificant resonant peaks can be modulated The maximummodulation depth reaches 988 at 747 THz [72]

223 Modulator Based on Flexible Substrate Flexible sub-strate was used to fabricate optical modulator with manymerits [104 105] as described in Section 213 For electricmodulator flexible substrate plays also an important role Forexample Kocabas et al presented a graphene metamaterialTHz modulator based on a flexible substrate (Figure 10(1))which consists of two large-area graphene electrodes trans-ferred onto THz transparent substrates with ionic liquidelectrolytes between them The bias can change electrostaticdoping of graphene which affects its optical properties Inthis case the maximum modulation depth reaches only 50at 01-14 THz working at a bias of 3V The modulation speed

was slow [73] Virtually thismodulator showed excellent flex-ibility which has not been deformed after being flexed manytimes Modulator based on flexible substrate can also realizea high modulation depth For example E Kaya et al havedemonstrated that polyvinyl chloride (PVC) and polyethy-lene (PE) were selected as flexible substrates (Figure 10(2))A multilayer graphene modulator was fabricated on thoseflexible substrates by chemical vapor deposition This devicecould fully modulate THz wave with a frequency range of02-15 THz at a low voltage of 34V [74] The comparison ofelectrically tuned modulation properties based on differentsubstrate materials is shown in Table 3

224 Summary The modulation properties of differentmaterials reported in literatures are given in Table 2Graphene can achieve modulation depth close to 100 andthe modulation speed in the order of kHz At present thereare few reports on flexible THz modulators since organicfunctional materials and flexible substrates cannot fully becompatible with micronanofabrication processes Based onreviewing the electric modulator it implied that electricmodulation device needs to miniaturize the structure to lowinsertion loss and to improve modulation speed which willbe the future direction

23 Photoelectric Hybrid Tuned THz Modulator Sections 21and 22 described the modulation methods using electricityor light separately If two methods can be combined togetherbetter modulation effects may be obtained The photoelectrichybrid modulation method is a method of synthesizing elec-tric and optical modulation Its basic principle is using pumplight excited carriers in a base semiconductor material In themeantime the motion direction of the carrier is adjusted byan external bias [108] For example an optoelectric hybridmodulator was exhibited by Q Li et al [75] Its structurewas shown in Figure 12 Si substrate was pumped with acontinuous laser of 532 nm to generate a large number ofelectron-hole pairs Under the drive of the concentrationdifference the photoelectrons in the Si diffused into thegraphene until the equilibrium state was established Astructure similar to a PN junction was formed between thetwomaterials When the bias was varied from 0 V to -3 V thepeak modulation of the time-domain signal peak achieved51 at optical pump power with 420 mW When the biaschanged from 0 V to -4 V the modulation amplitude depthbecame 83 Based on this structure Ran Jiang et al used a

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

4 Research

1

2

3

(a)

(a)

(a)

(b)

(b)

(b)

400

350

300

250

200

150

100

50

THz A

mpl

itude

(mV

)

THz waves

CuPc pentacene

organic material100 sim 5000 nm

Si substrate500 m

Laser Power (mW) Laser Power (mW)0 200 400 600 800 1000

Delay time (ps)

fitting

100 200 300 400 500 600

0 200 400 600 800 1000

10

08

06

04

02

00

10

08

06

04

02

Mod

ulat

ion

Dep

th

Nor

mal

ized

TH

z sig

nal (

au)

Bare Si25um

50um10um

Bare Si25um

50um10um

293 JcＧ2

147 JcＧ2

74 JcＧ2

23 JcＧ2

074 JcＧ2

60

600050004000300020001000

0minus1000minus2000minus3000minus4000minus5000minus6000

Die

lect

ric co

nsta

nts

04 06 08 10 12 14 16

Frequency (THz)

04 06 08 10 12 14 16

Frequency (THz)

1000

800

600

400

200

0

Real

Imaginary

Con

duct

ivity

(Ω-1cm

-1)

SiRT80 ∘C100 ∘C

120 ∘C140 ∘C160 ∘C

Figure 3Optically tunedTHzmodulator based on semiconductormaterials andmetamaterials Panel (1) (a) SEM images of the fabricated InSbgrating (b) Normalized THz signals as a function of delay time measured by OPTP under different pump laser fluences and the exponentialcurve fittings Reproduced from [42] Panel (2) (a) Schematic view of THzwave transmissionmeasurement for bilayer samples (b) Dielectricfunctions obtained from pentaceneSi thermally annealed at various temperatures Reproduced from [43] Panel (3) (a) Prototype and spatialconfiguration of the Si-nanotip-based spatial THz modulator (b) THz transmission amplitude (left) and the modulation depth (right) ofdifferent modulators as a function of the laser pumping power Reproduced from [44]

are the two-dimensional version ofmetamaterials Comparedto three-dimensional bulk metamaterials the thickness ofthe metasurfaces relative to the operating wavelength isnegligible They consume less physical space and have lowerinsertion loss Longqing Cong et al demonstrated an active

hybrid metasurface integrated with patterned semiconductorinclusions for all-optical active control of terahertzwaves [51]It achieved ultrafast modulation of the polarization state Byproperly incorporating silicon islands into the metamaterialunit cells Xieyu Chen et al presented a metasurface which

Research 5

Valence band

Dirac point

Conduction band

EHole

Electron

Fermi level Fermi level

Fermi level

(a) (b) (c) (d)

＋Ｓ

＋Ｒ

Figure 4 Schematic diagram of energy band structure of graphene (a) Band structure of graphene (b) Fermi level is at Dirac point (c) Fermilevel is in valence band (d) Fermi level is in conduction band

can be optically controlled with amodulation depth reaching68 and 62 for horizontal and vertical polarizations [52] Itopens up new avenues for the design of active metamaterials

212 Modulator Based on 2D Materials 2D materials haveunusual electrical and optical properties In recent years theyattracted increasing attention for applications in optoelec-tronics Graphene the best-known 2D material has beenwidely used for THz modulators tuned by light since beingdiscovered in 2004 [53] The carbon atom arrangementof graphene determines its unique conical band structureThe conductivity of graphene is contributed by the in-bandtransition of electrons and the transition between bandsIn the THz range the in-band transition of electrons playsa decisive factor due to the photon energy being smallTherefore we can approximate the thin layer conductivity ofgraphene by the Drude model as [54]

120590 (120596) = minus 119895119863120587 (120596 minus 119894Γ) (3)

119863 is the Drude weight and it is given as

119863 = V1198651198902ℏ (4)

V119865 = 119864119865ℏradic|119899| (5)

where Γ contributed to the acoustic phonon scattering V119865 isthe Fermi velocity 119899 is the carrier concentration

Thus the thin layer conductivity of graphene is closelyrelated to the Fermi level In general the concentration andtype of carriers can be dynamically modulated by changingthe Fermi level position of graphene Figure 4(a) is the bandstructure of grapheme [55 56] When the Fermi level is inthe conduction band the main carrier is free electrons whenthe Fermi level is in the valence band the main carrier isa hole when the Fermi level is at Dirac point the carrierconcentration is at a minimum and the conductivity ofgraphene is also very low

Graphene has low insertion losses which is extremelyideal for optical modulation Zhang Xiangrsquos team firstly

demonstrated it They proposed an optical modulator by reg-ulating the Fermi level of graphene [57] The most graphenemodulators consist of graphene and semiconductor materialto formheterojunction Laser beampenetrates Si to produce alarge number of carriers meanwhile photogenerated carriersdiffuse into the graphene layer and change its conductivitywhich results in an amount of THz wave absorption bygraphene layer The most classic work is light-modulatinggraphene devices formed with graphene on Si (GOS) demon-strated by Weis in 2012 (Figure 5(1)) This modulator waspumped by a 780 nm femtosecond laser The graphene layerabsorbed the modulated beam of approximately 23 Asexcited with wavelength of 750 nm and power of 40 mWpumped laser this modulator could realize the modulationabout 68 The tunable THz bandwidth was in the range of02-2 THz When the optical pump energy reached 500 mW(750 nm) the transmission THz wave almost completelydisappeared [58] By replacing Si with Ge an optical modula-tor of full modulation based on graphene was demonstrated(Figure 5(3)) A laser with a wavelength of 1550 nm could beused to pump effectively The measurement results presentedthat the modulation frequency range modulation depthand modulation speed were 025-1 THz 94 and 200kHz respectively [59] Although it achieved full modulationit required a high-power laser With development of 2Dmaterial fabrication an efficient THz modulator with lowpower was demonstrated It needs only an external 450 nmcontinuous wave laser It is mainly coated with a high-qualitymonolayer graphene filmon a Si substrateWith external lightexcitation the proposedmodulator could reach amodulationdepth of 74 Incidentally its modulation depth can befurther improved [60]

Monolayer transition metal dichalcogenides (TMDssuch as MoS2 and WS2) are direct-bandgap semiconductorsoffering properties complementary to graphene [89] A greatnumber of modulators based onmonolayer TMDs have beenreported For example a bidimensional material modulator(Figure 5(4)) based on MoS2 and Si improved modulationdepth as high as 649 at 09 THz pumped with an 808 nmlaser The modulation can be much higher after annealingMoS2 Under a larger pump power of 456 W this depth wasabout 96 These results suggested that MoS2 is a promising

6 Research

1

2 3 4

5 6

(a)

pow

er fl

ow graphene

losslesssilicon

attenuatedTHz-beam

infrared-beam

distance

THz Wave Si layer

THz wave

pump light

Detector

-299 nm

355 nm

Reflection

Reflection

CW Laser

CW Laser

Transmission

Transmission

OrganicSi

OrganicSi

THz

THz

THz Camera

THz-CWs

Si

Modulator

Si

808nmLaser

1550nm

Laser

EH

EH

siliconconducting

(b)

(b)(a) (c)

W３2

W３2 layer

10

08

06

04

02

00

0706

05

04

03

02

01

00

Tran

smiss

ion

07

06

05

04

03

02

01

Tran

smiss

ion

0 100 200 300 400 500

Modulation beam power (mW)0 100 200 300 400 500

Modulation beam power (mW)

Modulation beam power (mW)0 10 20 30 40 50

Modulation beam power (mW)0 10 20 30 40 50

SiliconGOSModel

Mod

ulat

ion

dept

h

08

06

04

02

00

Mod

ulat

ion

dept

h

SiliconGOSModel

Figure 5 Optically tuned THz modulator based on 2D materials Panel (1) (a) Schematic view of graphene on Si sample (b) Normalizedtransmission (left) and depth (right) from the three phthalocyanine structures Reproduced from [58] Panel (2) Layer structure of the THzmodulator based on WS2 and Si Reproduced from [61] Panel (3) The modulator consists of a single-layer graphene sheet on a germaniumsubstrate The beam of the THz wave is completely overlapped by the laser beam Reproduced from [59] Panel (4) A sketch map of theexperiment Reproduced from [62] Panel (5) Experimental setup of the THz-CW for measuring transmission and reflection Reproducedfrom [45] Panel (6) (a) The AFM image of liquid-exfoliated WS2 nanosheets (b) The image of the prepared WS2-Si sample (c) Thicknessdistribution map of WS2 film measured by white light interferometer Reproduced from [63]

material [62] Later on many studies have been reported forimprovement in terms of cost manufacturing process pumppower response speed and modulation depth Liu et alstudied a highly efficient activeTHzwavemodulator based onMoS2Ge structure Monolayers of MoS2 and graphene sam-ples were grown on n-doped Ge substrates by chemical vapordeposition (CVD) The light control bandwidth was greatlywidened to reach 26 THz [90] In addition to the expansionof the modulation bandwidth Fan et al demonstrated a THzmodulator (Figure 5(2)) based on p-type annealed tungstendisulfide (WS2) and high-resistivity Si (n-type) structureThismodulator presented a laser power-dependent modulationmechanism Ranging from 025 to 2 THz the modulationdepth reached 99when the pumping laser was 259Wcm2

[61] Other researchers have adopted new methods to realizesimilar modulators The size and thickness of WS2 film arecontrolled by innovatively depositing liquid-released WS2nanosheets on Si instead of CVDmethod (Figure 5(6)) Withraising the pump power the modulation depth continues toincrease eventually reaching 948 under 470mW [63]

213 Modulator Based on Flexible Substrate Compared torigid substrate THz modulators [91] flexible substrates havemany advantages such as transparency lightweight low costand consistent adhesion [9 92] which offer a new potentialfor THz modulator For example in 2006 D Y Khanet al proposed a THz modulation on flexible substrates(Figure 6(1)) They made single-crystal Si with a thickness

Research 7

(a)

(a)

A

Au

Polyimide

Polyimide

ESRRGaAs

GaAs

(b)

(b)

1

2

Bond elements to prestrainedelastomeric substrate

Fabricate thin ribbonSi device elements

stretchableSi devices

Si

PDMS

motherwafer

L+dL

L

Peel back elastomer flip over

A-

09

08

07

06

05

04

03

02

Tran

smiss

ion

04 06 08 10 12 14 16 18

Frequency (THz)

Pump Power0mW025mW05mW1mW2mW

3mW4mW6mW8mW

E

Figure 6 Optically tuned THz modulator based on flexible substrate Panel (1) Schematic illustration of the process for building stretchablesingle-crystal Si devices on elastomeric substrates Reproduced from [64] Panel (2) (a) Schematic of the modulator structure (b) Refractiveindex of the ferrofluid at different magnetic field intensities Reproduced from [65]

of 20 nm to 320 nm into a Si ribbon with a width of 5120583m to 50 120583m and a length of 15 mm When supported bya flexible substrate this wavy Si ribbon could be reversiblystretched or compressed under high horizontal stress withoutdamaging it Considering the electrical properties of Siribbon its electrical parameters can bemodified by changingthe surface shape of the flexible substrate [64] An opticallytuned metamaterial modulator on a flexible polymer sheetwhich had a frequency modulated in the THz range wasproposed by Liu et al Electric split-ring resonators (eSRRs)are attached to a thin polyimide layer and assembled into themodulator (Figure 6(2)) The optical excitation of the GaAspatch changed the response of the metamaterial The meta-material effectively adjusted the effective dielectric constantIn their experiment amodulation depth of 60was achievedin the frequency range of 11-18 THz This kind of flexible

device can be more widely used and created on nonplanarstructures for extensive applications in the future [65] Forcurrent studies high repeatability has not been obtained aftermultiple bending of the flexible substrate due tometal fatigueproperty [78 93]

214 Summary The modulation properties based on dif-ferent materials are summarized in Table 2 The materialdominates the modulation depth and speed of THz waverather than the light source Graphene and other 2Dmaterialscan achieve close to 100modulation depth withmodulationspeed reaching the order of MHz On the other hand moreexperimental works should be carried out to explore thosemodulators based on simulations The modulation efficiencyof optically tuned THz modulator based on flexible substratehas yet to be improved Lower insertion loss and more

8 Research

Table 2 The comparison of optically tuned modulation properties based on different materials

Work Description Year Frequency Modulation depth RefLibon et al Multiple quantum well 1999 02-1 THz 40 [34]Deng L et al InSb gratings 2013 15 THz 4670 [42]Peter W et al Graphene on Si (GOS) 2012 02-2 THz 99 [56]Wen Q Y et al Graphene on Ge 2014 025-1 THz 94 [57]Cao Y et al MoS2 2016 05- 15THz 96 [62]Fan ZY et al WS2 2017 025- 2 THz 99 [61]Hyung K Yet al C60 2014 05- 15THz 98 [43]He T et al AlClPc 2015 0-25 THz 99 [45]Shi Z et al Si Nanotip 2017 025-1 THz gt90 [44]

compact integration will be the future direction of opticallytuned THz modulator

22 Electrically Tuned THz Modulator The other route formodulating THz wave is electrically tuned THz modulatorUsing a mixed type-Itype-II GaAsAlAs multiple-quantum-well sample Libon et al have demonstrated firstly anelectrically controllable modulator in 2000 [94] Electricalmodulation is to control the concentration of electrons inthe substrate materials or structures by applying bias voltagewhich modulate the amplitude of the incident THz wave[94]The transmission bandwidth andmodulation depth andspeed are limited by dielectric constant loss and responsetime of the materials Consequently to achieve high-speedband-pass modulation or broadband filter it is necessaryto find high-speed low-loss THz functional materials anddesign novel structures

Similar to the main idea of optically tuned THz mod-ulator charge injection can also modulate the THz waveIn the past decade 2D electron gas (2DEG) in HEMTshas been demonstrated to modulate THz wave effectivelyHEMT is a field effect transistor that provides a 2DEGquantum well at the heterojunction interface of a highlydoped semiconductor (typically AlGaAs) and an originalundoped semiconductor (GaAs) In 2000 Kersting presenteda GaAsAlGaAs heterostructure device which powered upthe phase modulation of THz signals [94] This modulatorcontained five parabolic quantum wells (PQWs) THz wavemodulation can be realized by stimulating electrically low-energy electron to high-energy states Figure 7(1)(b) showsthe power spectrum of the differential modulation signaland the power spectrum of the incident pulse It led to thedevelopment of THz electronics chip due to a reduction ofthe device dimensions Nevertheless it cannot operate at lowtemperatures To allow THz modulator to be operated atroom temperature Kleine-Ostmann et al presented room-temperature THz wave modulator by using a device based ona gated 2DEG [66] This modulator mainly included a 2DEGwhose density could be controlled by the gate voltage Thismajor breakthrough illustrated that 2DEG in semiconductorscould be used to control THz wave effectively [95] Similarlyit has also been used for THz emission [96] THz detection[97] etc Other 2DEG systems can be also used to realizethe modulation for example a THz modulator based on

2DEGof a GaNAlGaN heterostructure was demonstrated byZhou (Figure 7(3))Themodulator had amaximum intensitymodulation depth of 93 and a 3dB operating bandwidth of400 kHz It required only a low driving voltage amplitude of2V under 87 K This active plasma-based THz modulatormay provide a promising solution in THz technology fieldsfor the metamaterial THz modulator [67]

221Modulator Based on Semiconductors andMetamaterialsIn addition metamaterials are also a hot spot in THzmodula-tor [98] Kebin Fan et al demonstrated a metamaterial activedevice which consists of gold split-ring resonators (SRRs) onGaAs thin film grown on Si substrate [8] It has achieved50 modulation depth and 100 kHz modulation speedSchottkyn-doped GaAs devices had been developed toincrease greatly the modulation speed In further studies themodulation speed had reached 10 MHz however the modu-lation depth of this modulator was only 30 [99]The single-particle nonresonant absorption mechanism described inDrude model has been used in the related studies As seenfrom experiments mentioned above Drude model has notyet been effective enough for the theory of metamaterialmodulator [100]

222 Modulator Based on Graphene To further improve themodulation depth and modulation speed of THz modulatordifferent active medium as substrate has been tested Therefractive index of those materials can usually be written inthe form of a complex refractive index 119899 = 119899119903 + 119894119899119894 where 119899119903is the real part of the refractive index and 119899119894 the imaginarypart of the refractive index When the voltage is adjustedthe real and imaginary parts are usually changed at the sametime By using those changes different electric modulatorscan be developed For example electroabsorption modulatoris based on imaginary part changing When the real partof the refractive index changed with applied voltage thetransmission phase of THz wave changed Therefore forthe electroabsorption modulator based on 2D material abetter modulation effect can be obtained by changing therefractive index through applied voltage In fact electromod-ulators based on 2D materials mainly focused on grapheneGraphene has many particular properties such as adjustablethin layer conductivity and long mean free path due to itsunique conical energy band structure [101ndash103] Its linear

Research 9

1

(a)

THz wave

THz wave

undoped

undoped

SI GaAs

GaAs

GaAsSi

Cr 5 nmCap 10 nmDoped Layer 50 nm

Spacer 7 nm2 DEG

Buffer 1m

Substrate 500 m

2

3

(a)

(a)

(b)

(b)

0

minus4

minus8

minus12

minus16

minus20

minus24

mod

ulat

ion

(pA

)

(a)

(b)

(b)

0 5 10 15 20

time t (ps)

+50 V+20 V+10 V-05 V-10 V-20 V-50 V-100 V

15

10

5

0

5 0 minus5

minus4 minus3 minus2 minus1 0

minus10gate voltage (V)

Gate voltage (V)

peak

-to-p

eak

ampl

itude

(pA

)

D

W

G A

Split gap

E

H

E

H

10

08

06

04

02

00

06

04

05

02

03

01

00

Tran

smiss

ion

Tran

smitt

ance

05 10 15 20

Frequency (THz)0 V1 V2 V4 V8 V16 V

VG

DS

2DEG Sapphire substrate

SI-GaAs

87K20K50K77K

100K120K150K170K

200K230K300K

Ｈ = 14times1018 ＝Ｇ-3

Figure 7 Electrically tuned THz modulator based on semiconductors and metamaterials Panel (1) (a) The structure of the THz modulator(b) Gate voltage dependence of the modulation signal (upper) Peak-to-peak amplitude of the modulator signal vs gate voltage (nether)Reproduced from [66] Panel (2) (a) Geometry and dimensions of the THz metamaterial switchmodulator (b) Frequency-dependenttransmitted intensity of THz radiation Reproduced from [8] Panel (3) (a) Three-dimensional schematic illustration of the plasmon-basedTHz modulator (b) Dependence of transmission on gate voltage at different temperatures Reproduced from [67]

10 Research

(a) (b)

(a)

(c)

(b)

1

2

3

(b)

(a)

Vg

Terahertzbeam

Terahertzbeam

ℎ]

Vg1 Vg2

EF2gtEF1 (Vg2gtVg1)EF1 (Vg1)

||2

10

08

06

04

02

00

Pow

er re

flect

ance

570 590 610 630

Frequency (GHz)

graphene

V

V = 0

２ rarr 1

２ rarr 0

６ = 0

top graphene layer bottom graphene layer

qV

10

08

06

04

02 07

06

0500

Tran

smitt

ance

Tran

smitt

ance

560

580

600

620

640

660

570 575 580 585

Frequency (GHz)Vg = 50 VVg = 0 V

Figure 8 Electrically tuned THz modulator based on graphene Panel (1) (a) Schematic of the proof-of-concept graphene THz modulator(b) Schematic of the THz modulator (c) Schematic of the device and electric field distribution when the substrate thickness and plasmonicresonance are matched to an odd multiple of a quarter wavelength of the THz wave Reproduced from [68] Panel (2) (a) Schematic of theTHz modulator (b) Normalized reflectance (RR (VCNP)) Reproduced from [69] Panel (3) (a) Schematic of the device and electric fielddistribution (b) Operation mechanism of the self-gated graphene pair Reproduced from [70]

dispersion relationship between energy and crystal momen-tum makes graphene superior to other semiconductor mate-rials

S Rodriguez utilized graphene to modulate THz wave(Figure 8(1)) The modulation depth and modulation speed

could reach 15 and 20 kHz respectively [68] Meanwhilethe same research group presented a reflective THz modu-lator as shown in Figure 8(2) The silver film on the backof the reflective THz modulator acted as both gate andmirror which improved the modulation depth with 64

Research 11

Table 3 The comparison of electrically tuned modulation properties based on different substrate materials

Work Description Year Frequency Modulation depth RefHuang Y D Plasmonic 2016 - 93 [67]Zhou G VO2 2017 03-10THz gt50 [100]Sensale-R B Graphene 2012 10THz 100 [106]Liang G Graphene 2015 3THz 94-100 [71]Huang Z Monolayer Graphene 2018 1-7THz gt85 [72]Chikhi N Liquid crystals 2018 1-3THz gt90 [107]Kaya E Flexible Substrate 2018 02-15THz 100 [74]

and expanded bandwidth ranging 057-063 THz comparedto their early works However its insertion loss and mod-ulation speed only reach 2 dB and 4 kHz respectively[69] The essence of those works is to change the Fermilevel of graphene by adjusting applied voltage The follow-ing graphene THz modulators with different structures arealso based on this method To improve modulation depthmultiple layers of graphene are a choice [102] As shown inFigure 8(3) a THz electroabsorption modulator based ontwo-layer graphene with periodic micron-belt pattern waspresented The THz wave was vertically illustrated on thesurface of graphene which enhanced the absorption of lightbecause of the plasma effect This modulator could operateat frequencies up to tens of THz and the modulation depthcould even reach 100 [103]

Usually the modulation depth of most modulators basedon monolayer graphene can only reach 80 [69] Howevermonolayer graphene combined with other structures ordevices can improve the modulation depth For exampleG Liang puts monolayer graphene on top of quantumcascade laser (QCL) to demonstrate modulation depth of100 (Figure 9(1)) and modulation speed of 110 MHz [71]The high modulation depth was a consequence of a stronginteraction between the graphene and THz wave whereasthe high modulation speed can be improved by reducingthe dimension of device It has recently been demonstratedthat a THz modulator can realize full modulation with awide modulation bandwidth and fast modulation speedUsing monolayer graphene Huang proposed an adjustablecomplementary ring resonator (Figure 9(2)) It was worthmentioning that in the range of 1-2 THz and 3-7 THz threesignificant resonant peaks can be modulated The maximummodulation depth reaches 988 at 747 THz [72]

223 Modulator Based on Flexible Substrate Flexible sub-strate was used to fabricate optical modulator with manymerits [104 105] as described in Section 213 For electricmodulator flexible substrate plays also an important role Forexample Kocabas et al presented a graphene metamaterialTHz modulator based on a flexible substrate (Figure 10(1))which consists of two large-area graphene electrodes trans-ferred onto THz transparent substrates with ionic liquidelectrolytes between them The bias can change electrostaticdoping of graphene which affects its optical properties Inthis case the maximum modulation depth reaches only 50at 01-14 THz working at a bias of 3V The modulation speed

was slow [73] Virtually thismodulator showed excellent flex-ibility which has not been deformed after being flexed manytimes Modulator based on flexible substrate can also realizea high modulation depth For example E Kaya et al havedemonstrated that polyvinyl chloride (PVC) and polyethy-lene (PE) were selected as flexible substrates (Figure 10(2))A multilayer graphene modulator was fabricated on thoseflexible substrates by chemical vapor deposition This devicecould fully modulate THz wave with a frequency range of02-15 THz at a low voltage of 34V [74] The comparison ofelectrically tuned modulation properties based on differentsubstrate materials is shown in Table 3

224 Summary The modulation properties of differentmaterials reported in literatures are given in Table 2Graphene can achieve modulation depth close to 100 andthe modulation speed in the order of kHz At present thereare few reports on flexible THz modulators since organicfunctional materials and flexible substrates cannot fully becompatible with micronanofabrication processes Based onreviewing the electric modulator it implied that electricmodulation device needs to miniaturize the structure to lowinsertion loss and to improve modulation speed which willbe the future direction

23 Photoelectric Hybrid Tuned THz Modulator Sections 21and 22 described the modulation methods using electricityor light separately If two methods can be combined togetherbetter modulation effects may be obtained The photoelectrichybrid modulation method is a method of synthesizing elec-tric and optical modulation Its basic principle is using pumplight excited carriers in a base semiconductor material In themeantime the motion direction of the carrier is adjusted byan external bias [108] For example an optoelectric hybridmodulator was exhibited by Q Li et al [75] Its structurewas shown in Figure 12 Si substrate was pumped with acontinuous laser of 532 nm to generate a large number ofelectron-hole pairs Under the drive of the concentrationdifference the photoelectrons in the Si diffused into thegraphene until the equilibrium state was established Astructure similar to a PN junction was formed between thetwomaterials When the bias was varied from 0 V to -3 V thepeak modulation of the time-domain signal peak achieved51 at optical pump power with 420 mW When the biaschanged from 0 V to -4 V the modulation amplitude depthbecame 83 Based on this structure Ran Jiang et al used a

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

Research 5

Valence band

Dirac point

Conduction band

EHole

Electron

Fermi level Fermi level

Fermi level

(a) (b) (c) (d)

＋Ｓ

＋Ｒ

Figure 4 Schematic diagram of energy band structure of graphene (a) Band structure of graphene (b) Fermi level is at Dirac point (c) Fermilevel is in valence band (d) Fermi level is in conduction band

can be optically controlled with amodulation depth reaching68 and 62 for horizontal and vertical polarizations [52] Itopens up new avenues for the design of active metamaterials

212 Modulator Based on 2D Materials 2D materials haveunusual electrical and optical properties In recent years theyattracted increasing attention for applications in optoelec-tronics Graphene the best-known 2D material has beenwidely used for THz modulators tuned by light since beingdiscovered in 2004 [53] The carbon atom arrangementof graphene determines its unique conical band structureThe conductivity of graphene is contributed by the in-bandtransition of electrons and the transition between bandsIn the THz range the in-band transition of electrons playsa decisive factor due to the photon energy being smallTherefore we can approximate the thin layer conductivity ofgraphene by the Drude model as [54]

120590 (120596) = minus 119895119863120587 (120596 minus 119894Γ) (3)

119863 is the Drude weight and it is given as

119863 = V1198651198902ℏ (4)

V119865 = 119864119865ℏradic|119899| (5)

where Γ contributed to the acoustic phonon scattering V119865 isthe Fermi velocity 119899 is the carrier concentration

Thus the thin layer conductivity of graphene is closelyrelated to the Fermi level In general the concentration andtype of carriers can be dynamically modulated by changingthe Fermi level position of graphene Figure 4(a) is the bandstructure of grapheme [55 56] When the Fermi level is inthe conduction band the main carrier is free electrons whenthe Fermi level is in the valence band the main carrier isa hole when the Fermi level is at Dirac point the carrierconcentration is at a minimum and the conductivity ofgraphene is also very low

Graphene has low insertion losses which is extremelyideal for optical modulation Zhang Xiangrsquos team firstly

demonstrated it They proposed an optical modulator by reg-ulating the Fermi level of graphene [57] The most graphenemodulators consist of graphene and semiconductor materialto formheterojunction Laser beampenetrates Si to produce alarge number of carriers meanwhile photogenerated carriersdiffuse into the graphene layer and change its conductivitywhich results in an amount of THz wave absorption bygraphene layer The most classic work is light-modulatinggraphene devices formed with graphene on Si (GOS) demon-strated by Weis in 2012 (Figure 5(1)) This modulator waspumped by a 780 nm femtosecond laser The graphene layerabsorbed the modulated beam of approximately 23 Asexcited with wavelength of 750 nm and power of 40 mWpumped laser this modulator could realize the modulationabout 68 The tunable THz bandwidth was in the range of02-2 THz When the optical pump energy reached 500 mW(750 nm) the transmission THz wave almost completelydisappeared [58] By replacing Si with Ge an optical modula-tor of full modulation based on graphene was demonstrated(Figure 5(3)) A laser with a wavelength of 1550 nm could beused to pump effectively The measurement results presentedthat the modulation frequency range modulation depthand modulation speed were 025-1 THz 94 and 200kHz respectively [59] Although it achieved full modulationit required a high-power laser With development of 2Dmaterial fabrication an efficient THz modulator with lowpower was demonstrated It needs only an external 450 nmcontinuous wave laser It is mainly coated with a high-qualitymonolayer graphene filmon a Si substrateWith external lightexcitation the proposedmodulator could reach amodulationdepth of 74 Incidentally its modulation depth can befurther improved [60]

Monolayer transition metal dichalcogenides (TMDssuch as MoS2 and WS2) are direct-bandgap semiconductorsoffering properties complementary to graphene [89] A greatnumber of modulators based onmonolayer TMDs have beenreported For example a bidimensional material modulator(Figure 5(4)) based on MoS2 and Si improved modulationdepth as high as 649 at 09 THz pumped with an 808 nmlaser The modulation can be much higher after annealingMoS2 Under a larger pump power of 456 W this depth wasabout 96 These results suggested that MoS2 is a promising

6 Research

1

2 3 4

5 6

(a)

pow

er fl

ow graphene

losslesssilicon

attenuatedTHz-beam

infrared-beam

distance

THz Wave Si layer

THz wave

pump light

Detector

-299 nm

355 nm

Reflection

Reflection

CW Laser

CW Laser

Transmission

Transmission

OrganicSi

OrganicSi

THz

THz

THz Camera

THz-CWs

Si

Modulator

Si

808nmLaser

1550nm

Laser

EH

EH

siliconconducting

(b)

(b)(a) (c)

W３2

W３2 layer

10

08

06

04

02

00

0706

05

04

03

02

01

00

Tran

smiss

ion

07

06

05

04

03

02

01

Tran

smiss

ion

0 100 200 300 400 500

Modulation beam power (mW)0 100 200 300 400 500

Modulation beam power (mW)

Modulation beam power (mW)0 10 20 30 40 50

Modulation beam power (mW)0 10 20 30 40 50

SiliconGOSModel

Mod

ulat

ion

dept

h

08

06

04

02

00

Mod

ulat

ion

dept

h

SiliconGOSModel

Figure 5 Optically tuned THz modulator based on 2D materials Panel (1) (a) Schematic view of graphene on Si sample (b) Normalizedtransmission (left) and depth (right) from the three phthalocyanine structures Reproduced from [58] Panel (2) Layer structure of the THzmodulator based on WS2 and Si Reproduced from [61] Panel (3) The modulator consists of a single-layer graphene sheet on a germaniumsubstrate The beam of the THz wave is completely overlapped by the laser beam Reproduced from [59] Panel (4) A sketch map of theexperiment Reproduced from [62] Panel (5) Experimental setup of the THz-CW for measuring transmission and reflection Reproducedfrom [45] Panel (6) (a) The AFM image of liquid-exfoliated WS2 nanosheets (b) The image of the prepared WS2-Si sample (c) Thicknessdistribution map of WS2 film measured by white light interferometer Reproduced from [63]

material [62] Later on many studies have been reported forimprovement in terms of cost manufacturing process pumppower response speed and modulation depth Liu et alstudied a highly efficient activeTHzwavemodulator based onMoS2Ge structure Monolayers of MoS2 and graphene sam-ples were grown on n-doped Ge substrates by chemical vapordeposition (CVD) The light control bandwidth was greatlywidened to reach 26 THz [90] In addition to the expansionof the modulation bandwidth Fan et al demonstrated a THzmodulator (Figure 5(2)) based on p-type annealed tungstendisulfide (WS2) and high-resistivity Si (n-type) structureThismodulator presented a laser power-dependent modulationmechanism Ranging from 025 to 2 THz the modulationdepth reached 99when the pumping laser was 259Wcm2

[61] Other researchers have adopted new methods to realizesimilar modulators The size and thickness of WS2 film arecontrolled by innovatively depositing liquid-released WS2nanosheets on Si instead of CVDmethod (Figure 5(6)) Withraising the pump power the modulation depth continues toincrease eventually reaching 948 under 470mW [63]

213 Modulator Based on Flexible Substrate Compared torigid substrate THz modulators [91] flexible substrates havemany advantages such as transparency lightweight low costand consistent adhesion [9 92] which offer a new potentialfor THz modulator For example in 2006 D Y Khanet al proposed a THz modulation on flexible substrates(Figure 6(1)) They made single-crystal Si with a thickness

Research 7

(a)

(a)

A

Au

Polyimide

Polyimide

ESRRGaAs

GaAs

(b)

(b)

1

2

Bond elements to prestrainedelastomeric substrate

Fabricate thin ribbonSi device elements

stretchableSi devices

Si

PDMS

motherwafer

L+dL

L

Peel back elastomer flip over

A-

09

08

07

06

05

04

03

02

Tran

smiss

ion

04 06 08 10 12 14 16 18

Frequency (THz)

Pump Power0mW025mW05mW1mW2mW

3mW4mW6mW8mW

E

Figure 6 Optically tuned THz modulator based on flexible substrate Panel (1) Schematic illustration of the process for building stretchablesingle-crystal Si devices on elastomeric substrates Reproduced from [64] Panel (2) (a) Schematic of the modulator structure (b) Refractiveindex of the ferrofluid at different magnetic field intensities Reproduced from [65]

of 20 nm to 320 nm into a Si ribbon with a width of 5120583m to 50 120583m and a length of 15 mm When supported bya flexible substrate this wavy Si ribbon could be reversiblystretched or compressed under high horizontal stress withoutdamaging it Considering the electrical properties of Siribbon its electrical parameters can bemodified by changingthe surface shape of the flexible substrate [64] An opticallytuned metamaterial modulator on a flexible polymer sheetwhich had a frequency modulated in the THz range wasproposed by Liu et al Electric split-ring resonators (eSRRs)are attached to a thin polyimide layer and assembled into themodulator (Figure 6(2)) The optical excitation of the GaAspatch changed the response of the metamaterial The meta-material effectively adjusted the effective dielectric constantIn their experiment amodulation depth of 60was achievedin the frequency range of 11-18 THz This kind of flexible

device can be more widely used and created on nonplanarstructures for extensive applications in the future [65] Forcurrent studies high repeatability has not been obtained aftermultiple bending of the flexible substrate due tometal fatigueproperty [78 93]

214 Summary The modulation properties based on dif-ferent materials are summarized in Table 2 The materialdominates the modulation depth and speed of THz waverather than the light source Graphene and other 2Dmaterialscan achieve close to 100modulation depth withmodulationspeed reaching the order of MHz On the other hand moreexperimental works should be carried out to explore thosemodulators based on simulations The modulation efficiencyof optically tuned THz modulator based on flexible substratehas yet to be improved Lower insertion loss and more

8 Research

Table 2 The comparison of optically tuned modulation properties based on different materials

Work Description Year Frequency Modulation depth RefLibon et al Multiple quantum well 1999 02-1 THz 40 [34]Deng L et al InSb gratings 2013 15 THz 4670 [42]Peter W et al Graphene on Si (GOS) 2012 02-2 THz 99 [56]Wen Q Y et al Graphene on Ge 2014 025-1 THz 94 [57]Cao Y et al MoS2 2016 05- 15THz 96 [62]Fan ZY et al WS2 2017 025- 2 THz 99 [61]Hyung K Yet al C60 2014 05- 15THz 98 [43]He T et al AlClPc 2015 0-25 THz 99 [45]Shi Z et al Si Nanotip 2017 025-1 THz gt90 [44]

compact integration will be the future direction of opticallytuned THz modulator

22 Electrically Tuned THz Modulator The other route formodulating THz wave is electrically tuned THz modulatorUsing a mixed type-Itype-II GaAsAlAs multiple-quantum-well sample Libon et al have demonstrated firstly anelectrically controllable modulator in 2000 [94] Electricalmodulation is to control the concentration of electrons inthe substrate materials or structures by applying bias voltagewhich modulate the amplitude of the incident THz wave[94]The transmission bandwidth andmodulation depth andspeed are limited by dielectric constant loss and responsetime of the materials Consequently to achieve high-speedband-pass modulation or broadband filter it is necessaryto find high-speed low-loss THz functional materials anddesign novel structures

Similar to the main idea of optically tuned THz mod-ulator charge injection can also modulate the THz waveIn the past decade 2D electron gas (2DEG) in HEMTshas been demonstrated to modulate THz wave effectivelyHEMT is a field effect transistor that provides a 2DEGquantum well at the heterojunction interface of a highlydoped semiconductor (typically AlGaAs) and an originalundoped semiconductor (GaAs) In 2000 Kersting presenteda GaAsAlGaAs heterostructure device which powered upthe phase modulation of THz signals [94] This modulatorcontained five parabolic quantum wells (PQWs) THz wavemodulation can be realized by stimulating electrically low-energy electron to high-energy states Figure 7(1)(b) showsthe power spectrum of the differential modulation signaland the power spectrum of the incident pulse It led to thedevelopment of THz electronics chip due to a reduction ofthe device dimensions Nevertheless it cannot operate at lowtemperatures To allow THz modulator to be operated atroom temperature Kleine-Ostmann et al presented room-temperature THz wave modulator by using a device based ona gated 2DEG [66] This modulator mainly included a 2DEGwhose density could be controlled by the gate voltage Thismajor breakthrough illustrated that 2DEG in semiconductorscould be used to control THz wave effectively [95] Similarlyit has also been used for THz emission [96] THz detection[97] etc Other 2DEG systems can be also used to realizethe modulation for example a THz modulator based on

2DEGof a GaNAlGaN heterostructure was demonstrated byZhou (Figure 7(3))Themodulator had amaximum intensitymodulation depth of 93 and a 3dB operating bandwidth of400 kHz It required only a low driving voltage amplitude of2V under 87 K This active plasma-based THz modulatormay provide a promising solution in THz technology fieldsfor the metamaterial THz modulator [67]

221Modulator Based on Semiconductors andMetamaterialsIn addition metamaterials are also a hot spot in THzmodula-tor [98] Kebin Fan et al demonstrated a metamaterial activedevice which consists of gold split-ring resonators (SRRs) onGaAs thin film grown on Si substrate [8] It has achieved50 modulation depth and 100 kHz modulation speedSchottkyn-doped GaAs devices had been developed toincrease greatly the modulation speed In further studies themodulation speed had reached 10 MHz however the modu-lation depth of this modulator was only 30 [99]The single-particle nonresonant absorption mechanism described inDrude model has been used in the related studies As seenfrom experiments mentioned above Drude model has notyet been effective enough for the theory of metamaterialmodulator [100]

222 Modulator Based on Graphene To further improve themodulation depth and modulation speed of THz modulatordifferent active medium as substrate has been tested Therefractive index of those materials can usually be written inthe form of a complex refractive index 119899 = 119899119903 + 119894119899119894 where 119899119903is the real part of the refractive index and 119899119894 the imaginarypart of the refractive index When the voltage is adjustedthe real and imaginary parts are usually changed at the sametime By using those changes different electric modulatorscan be developed For example electroabsorption modulatoris based on imaginary part changing When the real partof the refractive index changed with applied voltage thetransmission phase of THz wave changed Therefore forthe electroabsorption modulator based on 2D material abetter modulation effect can be obtained by changing therefractive index through applied voltage In fact electromod-ulators based on 2D materials mainly focused on grapheneGraphene has many particular properties such as adjustablethin layer conductivity and long mean free path due to itsunique conical energy band structure [101ndash103] Its linear

Research 9

1

(a)

THz wave

THz wave

undoped

undoped

SI GaAs

GaAs

GaAsSi

Cr 5 nmCap 10 nmDoped Layer 50 nm

Spacer 7 nm2 DEG

Buffer 1m

Substrate 500 m

2

3

(a)

(a)

(b)

(b)

0

minus4

minus8

minus12

minus16

minus20

minus24

mod

ulat

ion

(pA

)

(a)

(b)

(b)

0 5 10 15 20

time t (ps)

+50 V+20 V+10 V-05 V-10 V-20 V-50 V-100 V

15

10

5

0

5 0 minus5

minus4 minus3 minus2 minus1 0

minus10gate voltage (V)

Gate voltage (V)

peak

-to-p

eak

ampl

itude

(pA

)

D

W

G A

Split gap

E

H

E

H

10

08

06

04

02

00

06

04

05

02

03

01

00

Tran

smiss

ion

Tran

smitt

ance

05 10 15 20

Frequency (THz)0 V1 V2 V4 V8 V16 V

VG

DS

2DEG Sapphire substrate

SI-GaAs

87K20K50K77K

100K120K150K170K

200K230K300K

Ｈ = 14times1018 ＝Ｇ-3

Figure 7 Electrically tuned THz modulator based on semiconductors and metamaterials Panel (1) (a) The structure of the THz modulator(b) Gate voltage dependence of the modulation signal (upper) Peak-to-peak amplitude of the modulator signal vs gate voltage (nether)Reproduced from [66] Panel (2) (a) Geometry and dimensions of the THz metamaterial switchmodulator (b) Frequency-dependenttransmitted intensity of THz radiation Reproduced from [8] Panel (3) (a) Three-dimensional schematic illustration of the plasmon-basedTHz modulator (b) Dependence of transmission on gate voltage at different temperatures Reproduced from [67]

10 Research

(a) (b)

(a)

(c)

(b)

1

2

3

(b)

(a)

Vg

Terahertzbeam

Terahertzbeam

ℎ]

Vg1 Vg2

EF2gtEF1 (Vg2gtVg1)EF1 (Vg1)

||2

10

08

06

04

02

00

Pow

er re

flect

ance

570 590 610 630

Frequency (GHz)

graphene

V

V = 0

２ rarr 1

２ rarr 0

６ = 0

top graphene layer bottom graphene layer

qV

10

08

06

04

02 07

06

0500

Tran

smitt

ance

Tran

smitt

ance

560

580

600

620

640

660

570 575 580 585

Frequency (GHz)Vg = 50 VVg = 0 V

Figure 8 Electrically tuned THz modulator based on graphene Panel (1) (a) Schematic of the proof-of-concept graphene THz modulator(b) Schematic of the THz modulator (c) Schematic of the device and electric field distribution when the substrate thickness and plasmonicresonance are matched to an odd multiple of a quarter wavelength of the THz wave Reproduced from [68] Panel (2) (a) Schematic of theTHz modulator (b) Normalized reflectance (RR (VCNP)) Reproduced from [69] Panel (3) (a) Schematic of the device and electric fielddistribution (b) Operation mechanism of the self-gated graphene pair Reproduced from [70]

dispersion relationship between energy and crystal momen-tum makes graphene superior to other semiconductor mate-rials

S Rodriguez utilized graphene to modulate THz wave(Figure 8(1)) The modulation depth and modulation speed

could reach 15 and 20 kHz respectively [68] Meanwhilethe same research group presented a reflective THz modu-lator as shown in Figure 8(2) The silver film on the backof the reflective THz modulator acted as both gate andmirror which improved the modulation depth with 64

Research 11

Table 3 The comparison of electrically tuned modulation properties based on different substrate materials

Work Description Year Frequency Modulation depth RefHuang Y D Plasmonic 2016 - 93 [67]Zhou G VO2 2017 03-10THz gt50 [100]Sensale-R B Graphene 2012 10THz 100 [106]Liang G Graphene 2015 3THz 94-100 [71]Huang Z Monolayer Graphene 2018 1-7THz gt85 [72]Chikhi N Liquid crystals 2018 1-3THz gt90 [107]Kaya E Flexible Substrate 2018 02-15THz 100 [74]

and expanded bandwidth ranging 057-063 THz comparedto their early works However its insertion loss and mod-ulation speed only reach 2 dB and 4 kHz respectively[69] The essence of those works is to change the Fermilevel of graphene by adjusting applied voltage The follow-ing graphene THz modulators with different structures arealso based on this method To improve modulation depthmultiple layers of graphene are a choice [102] As shown inFigure 8(3) a THz electroabsorption modulator based ontwo-layer graphene with periodic micron-belt pattern waspresented The THz wave was vertically illustrated on thesurface of graphene which enhanced the absorption of lightbecause of the plasma effect This modulator could operateat frequencies up to tens of THz and the modulation depthcould even reach 100 [103]

Usually the modulation depth of most modulators basedon monolayer graphene can only reach 80 [69] Howevermonolayer graphene combined with other structures ordevices can improve the modulation depth For exampleG Liang puts monolayer graphene on top of quantumcascade laser (QCL) to demonstrate modulation depth of100 (Figure 9(1)) and modulation speed of 110 MHz [71]The high modulation depth was a consequence of a stronginteraction between the graphene and THz wave whereasthe high modulation speed can be improved by reducingthe dimension of device It has recently been demonstratedthat a THz modulator can realize full modulation with awide modulation bandwidth and fast modulation speedUsing monolayer graphene Huang proposed an adjustablecomplementary ring resonator (Figure 9(2)) It was worthmentioning that in the range of 1-2 THz and 3-7 THz threesignificant resonant peaks can be modulated The maximummodulation depth reaches 988 at 747 THz [72]

223 Modulator Based on Flexible Substrate Flexible sub-strate was used to fabricate optical modulator with manymerits [104 105] as described in Section 213 For electricmodulator flexible substrate plays also an important role Forexample Kocabas et al presented a graphene metamaterialTHz modulator based on a flexible substrate (Figure 10(1))which consists of two large-area graphene electrodes trans-ferred onto THz transparent substrates with ionic liquidelectrolytes between them The bias can change electrostaticdoping of graphene which affects its optical properties Inthis case the maximum modulation depth reaches only 50at 01-14 THz working at a bias of 3V The modulation speed

was slow [73] Virtually thismodulator showed excellent flex-ibility which has not been deformed after being flexed manytimes Modulator based on flexible substrate can also realizea high modulation depth For example E Kaya et al havedemonstrated that polyvinyl chloride (PVC) and polyethy-lene (PE) were selected as flexible substrates (Figure 10(2))A multilayer graphene modulator was fabricated on thoseflexible substrates by chemical vapor deposition This devicecould fully modulate THz wave with a frequency range of02-15 THz at a low voltage of 34V [74] The comparison ofelectrically tuned modulation properties based on differentsubstrate materials is shown in Table 3

224 Summary The modulation properties of differentmaterials reported in literatures are given in Table 2Graphene can achieve modulation depth close to 100 andthe modulation speed in the order of kHz At present thereare few reports on flexible THz modulators since organicfunctional materials and flexible substrates cannot fully becompatible with micronanofabrication processes Based onreviewing the electric modulator it implied that electricmodulation device needs to miniaturize the structure to lowinsertion loss and to improve modulation speed which willbe the future direction

23 Photoelectric Hybrid Tuned THz Modulator Sections 21and 22 described the modulation methods using electricityor light separately If two methods can be combined togetherbetter modulation effects may be obtained The photoelectrichybrid modulation method is a method of synthesizing elec-tric and optical modulation Its basic principle is using pumplight excited carriers in a base semiconductor material In themeantime the motion direction of the carrier is adjusted byan external bias [108] For example an optoelectric hybridmodulator was exhibited by Q Li et al [75] Its structurewas shown in Figure 12 Si substrate was pumped with acontinuous laser of 532 nm to generate a large number ofelectron-hole pairs Under the drive of the concentrationdifference the photoelectrons in the Si diffused into thegraphene until the equilibrium state was established Astructure similar to a PN junction was formed between thetwomaterials When the bias was varied from 0 V to -3 V thepeak modulation of the time-domain signal peak achieved51 at optical pump power with 420 mW When the biaschanged from 0 V to -4 V the modulation amplitude depthbecame 83 Based on this structure Ran Jiang et al used a

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

6 Research

1

2 3 4

5 6

(a)

pow

er fl

ow graphene

losslesssilicon

attenuatedTHz-beam

infrared-beam

distance

THz Wave Si layer

THz wave

pump light

Detector

-299 nm

355 nm

Reflection

Reflection

CW Laser

CW Laser

Transmission

Transmission

OrganicSi

OrganicSi

THz

THz

THz Camera

THz-CWs

Si

Modulator

Si

808nmLaser

1550nm

Laser

EH

EH

siliconconducting

(b)

(b)(a) (c)

W３2

W３2 layer

10

08

06

04

02

00

0706

05

04

03

02

01

00

Tran

smiss

ion

07

06

05

04

03

02

01

Tran

smiss

ion

0 100 200 300 400 500

Modulation beam power (mW)0 100 200 300 400 500

Modulation beam power (mW)

Modulation beam power (mW)0 10 20 30 40 50

Modulation beam power (mW)0 10 20 30 40 50

SiliconGOSModel

Mod

ulat

ion

dept

h

08

06

04

02

00

Mod

ulat

ion

dept

h

SiliconGOSModel

Figure 5 Optically tuned THz modulator based on 2D materials Panel (1) (a) Schematic view of graphene on Si sample (b) Normalizedtransmission (left) and depth (right) from the three phthalocyanine structures Reproduced from [58] Panel (2) Layer structure of the THzmodulator based on WS2 and Si Reproduced from [61] Panel (3) The modulator consists of a single-layer graphene sheet on a germaniumsubstrate The beam of the THz wave is completely overlapped by the laser beam Reproduced from [59] Panel (4) A sketch map of theexperiment Reproduced from [62] Panel (5) Experimental setup of the THz-CW for measuring transmission and reflection Reproducedfrom [45] Panel (6) (a) The AFM image of liquid-exfoliated WS2 nanosheets (b) The image of the prepared WS2-Si sample (c) Thicknessdistribution map of WS2 film measured by white light interferometer Reproduced from [63]

material [62] Later on many studies have been reported forimprovement in terms of cost manufacturing process pumppower response speed and modulation depth Liu et alstudied a highly efficient activeTHzwavemodulator based onMoS2Ge structure Monolayers of MoS2 and graphene sam-ples were grown on n-doped Ge substrates by chemical vapordeposition (CVD) The light control bandwidth was greatlywidened to reach 26 THz [90] In addition to the expansionof the modulation bandwidth Fan et al demonstrated a THzmodulator (Figure 5(2)) based on p-type annealed tungstendisulfide (WS2) and high-resistivity Si (n-type) structureThismodulator presented a laser power-dependent modulationmechanism Ranging from 025 to 2 THz the modulationdepth reached 99when the pumping laser was 259Wcm2

[61] Other researchers have adopted new methods to realizesimilar modulators The size and thickness of WS2 film arecontrolled by innovatively depositing liquid-released WS2nanosheets on Si instead of CVDmethod (Figure 5(6)) Withraising the pump power the modulation depth continues toincrease eventually reaching 948 under 470mW [63]

213 Modulator Based on Flexible Substrate Compared torigid substrate THz modulators [91] flexible substrates havemany advantages such as transparency lightweight low costand consistent adhesion [9 92] which offer a new potentialfor THz modulator For example in 2006 D Y Khanet al proposed a THz modulation on flexible substrates(Figure 6(1)) They made single-crystal Si with a thickness

Research 7

(a)

(a)

A

Au

Polyimide

Polyimide

ESRRGaAs

GaAs

(b)

(b)

1

2

Bond elements to prestrainedelastomeric substrate

Fabricate thin ribbonSi device elements

stretchableSi devices

Si

PDMS

motherwafer

L+dL

L

Peel back elastomer flip over

A-

09

08

07

06

05

04

03

02

Tran

smiss

ion

04 06 08 10 12 14 16 18

Frequency (THz)

Pump Power0mW025mW05mW1mW2mW

3mW4mW6mW8mW

E

Figure 6 Optically tuned THz modulator based on flexible substrate Panel (1) Schematic illustration of the process for building stretchablesingle-crystal Si devices on elastomeric substrates Reproduced from [64] Panel (2) (a) Schematic of the modulator structure (b) Refractiveindex of the ferrofluid at different magnetic field intensities Reproduced from [65]

of 20 nm to 320 nm into a Si ribbon with a width of 5120583m to 50 120583m and a length of 15 mm When supported bya flexible substrate this wavy Si ribbon could be reversiblystretched or compressed under high horizontal stress withoutdamaging it Considering the electrical properties of Siribbon its electrical parameters can bemodified by changingthe surface shape of the flexible substrate [64] An opticallytuned metamaterial modulator on a flexible polymer sheetwhich had a frequency modulated in the THz range wasproposed by Liu et al Electric split-ring resonators (eSRRs)are attached to a thin polyimide layer and assembled into themodulator (Figure 6(2)) The optical excitation of the GaAspatch changed the response of the metamaterial The meta-material effectively adjusted the effective dielectric constantIn their experiment amodulation depth of 60was achievedin the frequency range of 11-18 THz This kind of flexible

device can be more widely used and created on nonplanarstructures for extensive applications in the future [65] Forcurrent studies high repeatability has not been obtained aftermultiple bending of the flexible substrate due tometal fatigueproperty [78 93]

214 Summary The modulation properties based on dif-ferent materials are summarized in Table 2 The materialdominates the modulation depth and speed of THz waverather than the light source Graphene and other 2Dmaterialscan achieve close to 100modulation depth withmodulationspeed reaching the order of MHz On the other hand moreexperimental works should be carried out to explore thosemodulators based on simulations The modulation efficiencyof optically tuned THz modulator based on flexible substratehas yet to be improved Lower insertion loss and more

8 Research

Table 2 The comparison of optically tuned modulation properties based on different materials

Work Description Year Frequency Modulation depth RefLibon et al Multiple quantum well 1999 02-1 THz 40 [34]Deng L et al InSb gratings 2013 15 THz 4670 [42]Peter W et al Graphene on Si (GOS) 2012 02-2 THz 99 [56]Wen Q Y et al Graphene on Ge 2014 025-1 THz 94 [57]Cao Y et al MoS2 2016 05- 15THz 96 [62]Fan ZY et al WS2 2017 025- 2 THz 99 [61]Hyung K Yet al C60 2014 05- 15THz 98 [43]He T et al AlClPc 2015 0-25 THz 99 [45]Shi Z et al Si Nanotip 2017 025-1 THz gt90 [44]

compact integration will be the future direction of opticallytuned THz modulator

22 Electrically Tuned THz Modulator The other route formodulating THz wave is electrically tuned THz modulatorUsing a mixed type-Itype-II GaAsAlAs multiple-quantum-well sample Libon et al have demonstrated firstly anelectrically controllable modulator in 2000 [94] Electricalmodulation is to control the concentration of electrons inthe substrate materials or structures by applying bias voltagewhich modulate the amplitude of the incident THz wave[94]The transmission bandwidth andmodulation depth andspeed are limited by dielectric constant loss and responsetime of the materials Consequently to achieve high-speedband-pass modulation or broadband filter it is necessaryto find high-speed low-loss THz functional materials anddesign novel structures

Similar to the main idea of optically tuned THz mod-ulator charge injection can also modulate the THz waveIn the past decade 2D electron gas (2DEG) in HEMTshas been demonstrated to modulate THz wave effectivelyHEMT is a field effect transistor that provides a 2DEGquantum well at the heterojunction interface of a highlydoped semiconductor (typically AlGaAs) and an originalundoped semiconductor (GaAs) In 2000 Kersting presenteda GaAsAlGaAs heterostructure device which powered upthe phase modulation of THz signals [94] This modulatorcontained five parabolic quantum wells (PQWs) THz wavemodulation can be realized by stimulating electrically low-energy electron to high-energy states Figure 7(1)(b) showsthe power spectrum of the differential modulation signaland the power spectrum of the incident pulse It led to thedevelopment of THz electronics chip due to a reduction ofthe device dimensions Nevertheless it cannot operate at lowtemperatures To allow THz modulator to be operated atroom temperature Kleine-Ostmann et al presented room-temperature THz wave modulator by using a device based ona gated 2DEG [66] This modulator mainly included a 2DEGwhose density could be controlled by the gate voltage Thismajor breakthrough illustrated that 2DEG in semiconductorscould be used to control THz wave effectively [95] Similarlyit has also been used for THz emission [96] THz detection[97] etc Other 2DEG systems can be also used to realizethe modulation for example a THz modulator based on

2DEGof a GaNAlGaN heterostructure was demonstrated byZhou (Figure 7(3))Themodulator had amaximum intensitymodulation depth of 93 and a 3dB operating bandwidth of400 kHz It required only a low driving voltage amplitude of2V under 87 K This active plasma-based THz modulatormay provide a promising solution in THz technology fieldsfor the metamaterial THz modulator [67]

221Modulator Based on Semiconductors andMetamaterialsIn addition metamaterials are also a hot spot in THzmodula-tor [98] Kebin Fan et al demonstrated a metamaterial activedevice which consists of gold split-ring resonators (SRRs) onGaAs thin film grown on Si substrate [8] It has achieved50 modulation depth and 100 kHz modulation speedSchottkyn-doped GaAs devices had been developed toincrease greatly the modulation speed In further studies themodulation speed had reached 10 MHz however the modu-lation depth of this modulator was only 30 [99]The single-particle nonresonant absorption mechanism described inDrude model has been used in the related studies As seenfrom experiments mentioned above Drude model has notyet been effective enough for the theory of metamaterialmodulator [100]

222 Modulator Based on Graphene To further improve themodulation depth and modulation speed of THz modulatordifferent active medium as substrate has been tested Therefractive index of those materials can usually be written inthe form of a complex refractive index 119899 = 119899119903 + 119894119899119894 where 119899119903is the real part of the refractive index and 119899119894 the imaginarypart of the refractive index When the voltage is adjustedthe real and imaginary parts are usually changed at the sametime By using those changes different electric modulatorscan be developed For example electroabsorption modulatoris based on imaginary part changing When the real partof the refractive index changed with applied voltage thetransmission phase of THz wave changed Therefore forthe electroabsorption modulator based on 2D material abetter modulation effect can be obtained by changing therefractive index through applied voltage In fact electromod-ulators based on 2D materials mainly focused on grapheneGraphene has many particular properties such as adjustablethin layer conductivity and long mean free path due to itsunique conical energy band structure [101ndash103] Its linear

Research 9

1

(a)

THz wave

THz wave

undoped

undoped

SI GaAs

GaAs

GaAsSi

Cr 5 nmCap 10 nmDoped Layer 50 nm

Spacer 7 nm2 DEG

Buffer 1m

Substrate 500 m

2

3

(a)

(a)

(b)

(b)

0

minus4

minus8

minus12

minus16

minus20

minus24

mod

ulat

ion

(pA

)

(a)

(b)

(b)

0 5 10 15 20

time t (ps)

+50 V+20 V+10 V-05 V-10 V-20 V-50 V-100 V

15

10

5

0

5 0 minus5

minus4 minus3 minus2 minus1 0

minus10gate voltage (V)

Gate voltage (V)

peak

-to-p

eak

ampl

itude

(pA

)

D

W

G A

Split gap

E

H

E

H

10

08

06

04

02

00

06

04

05

02

03

01

00

Tran

smiss

ion

Tran

smitt

ance

05 10 15 20

Frequency (THz)0 V1 V2 V4 V8 V16 V

VG

DS

2DEG Sapphire substrate

SI-GaAs

87K20K50K77K

100K120K150K170K

200K230K300K

Ｈ = 14times1018 ＝Ｇ-3

Figure 7 Electrically tuned THz modulator based on semiconductors and metamaterials Panel (1) (a) The structure of the THz modulator(b) Gate voltage dependence of the modulation signal (upper) Peak-to-peak amplitude of the modulator signal vs gate voltage (nether)Reproduced from [66] Panel (2) (a) Geometry and dimensions of the THz metamaterial switchmodulator (b) Frequency-dependenttransmitted intensity of THz radiation Reproduced from [8] Panel (3) (a) Three-dimensional schematic illustration of the plasmon-basedTHz modulator (b) Dependence of transmission on gate voltage at different temperatures Reproduced from [67]

10 Research

(a) (b)

(a)

(c)

(b)

1

2

3

(b)

(a)

Vg

Terahertzbeam

Terahertzbeam

ℎ]

Vg1 Vg2

EF2gtEF1 (Vg2gtVg1)EF1 (Vg1)

||2

10

08

06

04

02

00

Pow

er re

flect

ance

570 590 610 630

Frequency (GHz)

graphene

V

V = 0

２ rarr 1

２ rarr 0

６ = 0

top graphene layer bottom graphene layer

qV

10

08

06

04

02 07

06

0500

Tran

smitt

ance

Tran

smitt

ance

560

580

600

620

640

660

570 575 580 585

Frequency (GHz)Vg = 50 VVg = 0 V

Figure 8 Electrically tuned THz modulator based on graphene Panel (1) (a) Schematic of the proof-of-concept graphene THz modulator(b) Schematic of the THz modulator (c) Schematic of the device and electric field distribution when the substrate thickness and plasmonicresonance are matched to an odd multiple of a quarter wavelength of the THz wave Reproduced from [68] Panel (2) (a) Schematic of theTHz modulator (b) Normalized reflectance (RR (VCNP)) Reproduced from [69] Panel (3) (a) Schematic of the device and electric fielddistribution (b) Operation mechanism of the self-gated graphene pair Reproduced from [70]

dispersion relationship between energy and crystal momen-tum makes graphene superior to other semiconductor mate-rials

S Rodriguez utilized graphene to modulate THz wave(Figure 8(1)) The modulation depth and modulation speed

could reach 15 and 20 kHz respectively [68] Meanwhilethe same research group presented a reflective THz modu-lator as shown in Figure 8(2) The silver film on the backof the reflective THz modulator acted as both gate andmirror which improved the modulation depth with 64

Research 11

Table 3 The comparison of electrically tuned modulation properties based on different substrate materials

Work Description Year Frequency Modulation depth RefHuang Y D Plasmonic 2016 - 93 [67]Zhou G VO2 2017 03-10THz gt50 [100]Sensale-R B Graphene 2012 10THz 100 [106]Liang G Graphene 2015 3THz 94-100 [71]Huang Z Monolayer Graphene 2018 1-7THz gt85 [72]Chikhi N Liquid crystals 2018 1-3THz gt90 [107]Kaya E Flexible Substrate 2018 02-15THz 100 [74]

and expanded bandwidth ranging 057-063 THz comparedto their early works However its insertion loss and mod-ulation speed only reach 2 dB and 4 kHz respectively[69] The essence of those works is to change the Fermilevel of graphene by adjusting applied voltage The follow-ing graphene THz modulators with different structures arealso based on this method To improve modulation depthmultiple layers of graphene are a choice [102] As shown inFigure 8(3) a THz electroabsorption modulator based ontwo-layer graphene with periodic micron-belt pattern waspresented The THz wave was vertically illustrated on thesurface of graphene which enhanced the absorption of lightbecause of the plasma effect This modulator could operateat frequencies up to tens of THz and the modulation depthcould even reach 100 [103]

Usually the modulation depth of most modulators basedon monolayer graphene can only reach 80 [69] Howevermonolayer graphene combined with other structures ordevices can improve the modulation depth For exampleG Liang puts monolayer graphene on top of quantumcascade laser (QCL) to demonstrate modulation depth of100 (Figure 9(1)) and modulation speed of 110 MHz [71]The high modulation depth was a consequence of a stronginteraction between the graphene and THz wave whereasthe high modulation speed can be improved by reducingthe dimension of device It has recently been demonstratedthat a THz modulator can realize full modulation with awide modulation bandwidth and fast modulation speedUsing monolayer graphene Huang proposed an adjustablecomplementary ring resonator (Figure 9(2)) It was worthmentioning that in the range of 1-2 THz and 3-7 THz threesignificant resonant peaks can be modulated The maximummodulation depth reaches 988 at 747 THz [72]

223 Modulator Based on Flexible Substrate Flexible sub-strate was used to fabricate optical modulator with manymerits [104 105] as described in Section 213 For electricmodulator flexible substrate plays also an important role Forexample Kocabas et al presented a graphene metamaterialTHz modulator based on a flexible substrate (Figure 10(1))which consists of two large-area graphene electrodes trans-ferred onto THz transparent substrates with ionic liquidelectrolytes between them The bias can change electrostaticdoping of graphene which affects its optical properties Inthis case the maximum modulation depth reaches only 50at 01-14 THz working at a bias of 3V The modulation speed

was slow [73] Virtually thismodulator showed excellent flex-ibility which has not been deformed after being flexed manytimes Modulator based on flexible substrate can also realizea high modulation depth For example E Kaya et al havedemonstrated that polyvinyl chloride (PVC) and polyethy-lene (PE) were selected as flexible substrates (Figure 10(2))A multilayer graphene modulator was fabricated on thoseflexible substrates by chemical vapor deposition This devicecould fully modulate THz wave with a frequency range of02-15 THz at a low voltage of 34V [74] The comparison ofelectrically tuned modulation properties based on differentsubstrate materials is shown in Table 3

224 Summary The modulation properties of differentmaterials reported in literatures are given in Table 2Graphene can achieve modulation depth close to 100 andthe modulation speed in the order of kHz At present thereare few reports on flexible THz modulators since organicfunctional materials and flexible substrates cannot fully becompatible with micronanofabrication processes Based onreviewing the electric modulator it implied that electricmodulation device needs to miniaturize the structure to lowinsertion loss and to improve modulation speed which willbe the future direction

23 Photoelectric Hybrid Tuned THz Modulator Sections 21and 22 described the modulation methods using electricityor light separately If two methods can be combined togetherbetter modulation effects may be obtained The photoelectrichybrid modulation method is a method of synthesizing elec-tric and optical modulation Its basic principle is using pumplight excited carriers in a base semiconductor material In themeantime the motion direction of the carrier is adjusted byan external bias [108] For example an optoelectric hybridmodulator was exhibited by Q Li et al [75] Its structurewas shown in Figure 12 Si substrate was pumped with acontinuous laser of 532 nm to generate a large number ofelectron-hole pairs Under the drive of the concentrationdifference the photoelectrons in the Si diffused into thegraphene until the equilibrium state was established Astructure similar to a PN junction was formed between thetwomaterials When the bias was varied from 0 V to -3 V thepeak modulation of the time-domain signal peak achieved51 at optical pump power with 420 mW When the biaschanged from 0 V to -4 V the modulation amplitude depthbecame 83 Based on this structure Ran Jiang et al used a

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

Research 7

(a)

(a)

A

Au

Polyimide

Polyimide

ESRRGaAs

GaAs

(b)

(b)

1

2

Bond elements to prestrainedelastomeric substrate

Fabricate thin ribbonSi device elements

stretchableSi devices

Si

PDMS

motherwafer

L+dL

L

Peel back elastomer flip over

A-

09

08

07

06

05

04

03

02

Tran

smiss

ion

04 06 08 10 12 14 16 18

Frequency (THz)

Pump Power0mW025mW05mW1mW2mW

3mW4mW6mW8mW

E

Figure 6 Optically tuned THz modulator based on flexible substrate Panel (1) Schematic illustration of the process for building stretchablesingle-crystal Si devices on elastomeric substrates Reproduced from [64] Panel (2) (a) Schematic of the modulator structure (b) Refractiveindex of the ferrofluid at different magnetic field intensities Reproduced from [65]

of 20 nm to 320 nm into a Si ribbon with a width of 5120583m to 50 120583m and a length of 15 mm When supported bya flexible substrate this wavy Si ribbon could be reversiblystretched or compressed under high horizontal stress withoutdamaging it Considering the electrical properties of Siribbon its electrical parameters can bemodified by changingthe surface shape of the flexible substrate [64] An opticallytuned metamaterial modulator on a flexible polymer sheetwhich had a frequency modulated in the THz range wasproposed by Liu et al Electric split-ring resonators (eSRRs)are attached to a thin polyimide layer and assembled into themodulator (Figure 6(2)) The optical excitation of the GaAspatch changed the response of the metamaterial The meta-material effectively adjusted the effective dielectric constantIn their experiment amodulation depth of 60was achievedin the frequency range of 11-18 THz This kind of flexible

device can be more widely used and created on nonplanarstructures for extensive applications in the future [65] Forcurrent studies high repeatability has not been obtained aftermultiple bending of the flexible substrate due tometal fatigueproperty [78 93]

214 Summary The modulation properties based on dif-ferent materials are summarized in Table 2 The materialdominates the modulation depth and speed of THz waverather than the light source Graphene and other 2Dmaterialscan achieve close to 100modulation depth withmodulationspeed reaching the order of MHz On the other hand moreexperimental works should be carried out to explore thosemodulators based on simulations The modulation efficiencyof optically tuned THz modulator based on flexible substratehas yet to be improved Lower insertion loss and more

8 Research

Table 2 The comparison of optically tuned modulation properties based on different materials

Work Description Year Frequency Modulation depth RefLibon et al Multiple quantum well 1999 02-1 THz 40 [34]Deng L et al InSb gratings 2013 15 THz 4670 [42]Peter W et al Graphene on Si (GOS) 2012 02-2 THz 99 [56]Wen Q Y et al Graphene on Ge 2014 025-1 THz 94 [57]Cao Y et al MoS2 2016 05- 15THz 96 [62]Fan ZY et al WS2 2017 025- 2 THz 99 [61]Hyung K Yet al C60 2014 05- 15THz 98 [43]He T et al AlClPc 2015 0-25 THz 99 [45]Shi Z et al Si Nanotip 2017 025-1 THz gt90 [44]

compact integration will be the future direction of opticallytuned THz modulator

22 Electrically Tuned THz Modulator The other route formodulating THz wave is electrically tuned THz modulatorUsing a mixed type-Itype-II GaAsAlAs multiple-quantum-well sample Libon et al have demonstrated firstly anelectrically controllable modulator in 2000 [94] Electricalmodulation is to control the concentration of electrons inthe substrate materials or structures by applying bias voltagewhich modulate the amplitude of the incident THz wave[94]The transmission bandwidth andmodulation depth andspeed are limited by dielectric constant loss and responsetime of the materials Consequently to achieve high-speedband-pass modulation or broadband filter it is necessaryto find high-speed low-loss THz functional materials anddesign novel structures

Similar to the main idea of optically tuned THz mod-ulator charge injection can also modulate the THz waveIn the past decade 2D electron gas (2DEG) in HEMTshas been demonstrated to modulate THz wave effectivelyHEMT is a field effect transistor that provides a 2DEGquantum well at the heterojunction interface of a highlydoped semiconductor (typically AlGaAs) and an originalundoped semiconductor (GaAs) In 2000 Kersting presenteda GaAsAlGaAs heterostructure device which powered upthe phase modulation of THz signals [94] This modulatorcontained five parabolic quantum wells (PQWs) THz wavemodulation can be realized by stimulating electrically low-energy electron to high-energy states Figure 7(1)(b) showsthe power spectrum of the differential modulation signaland the power spectrum of the incident pulse It led to thedevelopment of THz electronics chip due to a reduction ofthe device dimensions Nevertheless it cannot operate at lowtemperatures To allow THz modulator to be operated atroom temperature Kleine-Ostmann et al presented room-temperature THz wave modulator by using a device based ona gated 2DEG [66] This modulator mainly included a 2DEGwhose density could be controlled by the gate voltage Thismajor breakthrough illustrated that 2DEG in semiconductorscould be used to control THz wave effectively [95] Similarlyit has also been used for THz emission [96] THz detection[97] etc Other 2DEG systems can be also used to realizethe modulation for example a THz modulator based on

2DEGof a GaNAlGaN heterostructure was demonstrated byZhou (Figure 7(3))Themodulator had amaximum intensitymodulation depth of 93 and a 3dB operating bandwidth of400 kHz It required only a low driving voltage amplitude of2V under 87 K This active plasma-based THz modulatormay provide a promising solution in THz technology fieldsfor the metamaterial THz modulator [67]

221Modulator Based on Semiconductors andMetamaterialsIn addition metamaterials are also a hot spot in THzmodula-tor [98] Kebin Fan et al demonstrated a metamaterial activedevice which consists of gold split-ring resonators (SRRs) onGaAs thin film grown on Si substrate [8] It has achieved50 modulation depth and 100 kHz modulation speedSchottkyn-doped GaAs devices had been developed toincrease greatly the modulation speed In further studies themodulation speed had reached 10 MHz however the modu-lation depth of this modulator was only 30 [99]The single-particle nonresonant absorption mechanism described inDrude model has been used in the related studies As seenfrom experiments mentioned above Drude model has notyet been effective enough for the theory of metamaterialmodulator [100]

222 Modulator Based on Graphene To further improve themodulation depth and modulation speed of THz modulatordifferent active medium as substrate has been tested Therefractive index of those materials can usually be written inthe form of a complex refractive index 119899 = 119899119903 + 119894119899119894 where 119899119903is the real part of the refractive index and 119899119894 the imaginarypart of the refractive index When the voltage is adjustedthe real and imaginary parts are usually changed at the sametime By using those changes different electric modulatorscan be developed For example electroabsorption modulatoris based on imaginary part changing When the real partof the refractive index changed with applied voltage thetransmission phase of THz wave changed Therefore forthe electroabsorption modulator based on 2D material abetter modulation effect can be obtained by changing therefractive index through applied voltage In fact electromod-ulators based on 2D materials mainly focused on grapheneGraphene has many particular properties such as adjustablethin layer conductivity and long mean free path due to itsunique conical energy band structure [101ndash103] Its linear

Research 9

1

(a)

THz wave

THz wave

undoped

undoped

SI GaAs

GaAs

GaAsSi

Cr 5 nmCap 10 nmDoped Layer 50 nm

Spacer 7 nm2 DEG

Buffer 1m

Substrate 500 m

2

3

(a)

(a)

(b)

(b)

0

minus4

minus8

minus12

minus16

minus20

minus24

mod

ulat

ion

(pA

)

(a)

(b)

(b)

0 5 10 15 20

time t (ps)

+50 V+20 V+10 V-05 V-10 V-20 V-50 V-100 V

15

10

5

0

5 0 minus5

minus4 minus3 minus2 minus1 0

minus10gate voltage (V)

Gate voltage (V)

peak

-to-p

eak

ampl

itude

(pA

)

D

W

G A

Split gap

E

H

E

H

10

08

06

04

02

00

06

04

05

02

03

01

00

Tran

smiss

ion

Tran

smitt

ance

05 10 15 20

Frequency (THz)0 V1 V2 V4 V8 V16 V

VG

DS

2DEG Sapphire substrate

SI-GaAs

87K20K50K77K

100K120K150K170K

200K230K300K

Ｈ = 14times1018 ＝Ｇ-3

Figure 7 Electrically tuned THz modulator based on semiconductors and metamaterials Panel (1) (a) The structure of the THz modulator(b) Gate voltage dependence of the modulation signal (upper) Peak-to-peak amplitude of the modulator signal vs gate voltage (nether)Reproduced from [66] Panel (2) (a) Geometry and dimensions of the THz metamaterial switchmodulator (b) Frequency-dependenttransmitted intensity of THz radiation Reproduced from [8] Panel (3) (a) Three-dimensional schematic illustration of the plasmon-basedTHz modulator (b) Dependence of transmission on gate voltage at different temperatures Reproduced from [67]

10 Research

(a) (b)

(a)

(c)

(b)

1

2

3

(b)

(a)

Vg

Terahertzbeam

Terahertzbeam

ℎ]

Vg1 Vg2

EF2gtEF1 (Vg2gtVg1)EF1 (Vg1)

||2

10

08

06

04

02

00

Pow

er re

flect

ance

570 590 610 630

Frequency (GHz)

graphene

V

V = 0

２ rarr 1

２ rarr 0

６ = 0

top graphene layer bottom graphene layer

qV

10

08

06

04

02 07

06

0500

Tran

smitt

ance

Tran

smitt

ance

560

580

600

620

640

660

570 575 580 585

Frequency (GHz)Vg = 50 VVg = 0 V

Figure 8 Electrically tuned THz modulator based on graphene Panel (1) (a) Schematic of the proof-of-concept graphene THz modulator(b) Schematic of the THz modulator (c) Schematic of the device and electric field distribution when the substrate thickness and plasmonicresonance are matched to an odd multiple of a quarter wavelength of the THz wave Reproduced from [68] Panel (2) (a) Schematic of theTHz modulator (b) Normalized reflectance (RR (VCNP)) Reproduced from [69] Panel (3) (a) Schematic of the device and electric fielddistribution (b) Operation mechanism of the self-gated graphene pair Reproduced from [70]

dispersion relationship between energy and crystal momen-tum makes graphene superior to other semiconductor mate-rials

S Rodriguez utilized graphene to modulate THz wave(Figure 8(1)) The modulation depth and modulation speed

could reach 15 and 20 kHz respectively [68] Meanwhilethe same research group presented a reflective THz modu-lator as shown in Figure 8(2) The silver film on the backof the reflective THz modulator acted as both gate andmirror which improved the modulation depth with 64

Research 11

Table 3 The comparison of electrically tuned modulation properties based on different substrate materials

Work Description Year Frequency Modulation depth RefHuang Y D Plasmonic 2016 - 93 [67]Zhou G VO2 2017 03-10THz gt50 [100]Sensale-R B Graphene 2012 10THz 100 [106]Liang G Graphene 2015 3THz 94-100 [71]Huang Z Monolayer Graphene 2018 1-7THz gt85 [72]Chikhi N Liquid crystals 2018 1-3THz gt90 [107]Kaya E Flexible Substrate 2018 02-15THz 100 [74]

and expanded bandwidth ranging 057-063 THz comparedto their early works However its insertion loss and mod-ulation speed only reach 2 dB and 4 kHz respectively[69] The essence of those works is to change the Fermilevel of graphene by adjusting applied voltage The follow-ing graphene THz modulators with different structures arealso based on this method To improve modulation depthmultiple layers of graphene are a choice [102] As shown inFigure 8(3) a THz electroabsorption modulator based ontwo-layer graphene with periodic micron-belt pattern waspresented The THz wave was vertically illustrated on thesurface of graphene which enhanced the absorption of lightbecause of the plasma effect This modulator could operateat frequencies up to tens of THz and the modulation depthcould even reach 100 [103]

Usually the modulation depth of most modulators basedon monolayer graphene can only reach 80 [69] Howevermonolayer graphene combined with other structures ordevices can improve the modulation depth For exampleG Liang puts monolayer graphene on top of quantumcascade laser (QCL) to demonstrate modulation depth of100 (Figure 9(1)) and modulation speed of 110 MHz [71]The high modulation depth was a consequence of a stronginteraction between the graphene and THz wave whereasthe high modulation speed can be improved by reducingthe dimension of device It has recently been demonstratedthat a THz modulator can realize full modulation with awide modulation bandwidth and fast modulation speedUsing monolayer graphene Huang proposed an adjustablecomplementary ring resonator (Figure 9(2)) It was worthmentioning that in the range of 1-2 THz and 3-7 THz threesignificant resonant peaks can be modulated The maximummodulation depth reaches 988 at 747 THz [72]

223 Modulator Based on Flexible Substrate Flexible sub-strate was used to fabricate optical modulator with manymerits [104 105] as described in Section 213 For electricmodulator flexible substrate plays also an important role Forexample Kocabas et al presented a graphene metamaterialTHz modulator based on a flexible substrate (Figure 10(1))which consists of two large-area graphene electrodes trans-ferred onto THz transparent substrates with ionic liquidelectrolytes between them The bias can change electrostaticdoping of graphene which affects its optical properties Inthis case the maximum modulation depth reaches only 50at 01-14 THz working at a bias of 3V The modulation speed

was slow [73] Virtually thismodulator showed excellent flex-ibility which has not been deformed after being flexed manytimes Modulator based on flexible substrate can also realizea high modulation depth For example E Kaya et al havedemonstrated that polyvinyl chloride (PVC) and polyethy-lene (PE) were selected as flexible substrates (Figure 10(2))A multilayer graphene modulator was fabricated on thoseflexible substrates by chemical vapor deposition This devicecould fully modulate THz wave with a frequency range of02-15 THz at a low voltage of 34V [74] The comparison ofelectrically tuned modulation properties based on differentsubstrate materials is shown in Table 3

224 Summary The modulation properties of differentmaterials reported in literatures are given in Table 2Graphene can achieve modulation depth close to 100 andthe modulation speed in the order of kHz At present thereare few reports on flexible THz modulators since organicfunctional materials and flexible substrates cannot fully becompatible with micronanofabrication processes Based onreviewing the electric modulator it implied that electricmodulation device needs to miniaturize the structure to lowinsertion loss and to improve modulation speed which willbe the future direction

23 Photoelectric Hybrid Tuned THz Modulator Sections 21and 22 described the modulation methods using electricityor light separately If two methods can be combined togetherbetter modulation effects may be obtained The photoelectrichybrid modulation method is a method of synthesizing elec-tric and optical modulation Its basic principle is using pumplight excited carriers in a base semiconductor material In themeantime the motion direction of the carrier is adjusted byan external bias [108] For example an optoelectric hybridmodulator was exhibited by Q Li et al [75] Its structurewas shown in Figure 12 Si substrate was pumped with acontinuous laser of 532 nm to generate a large number ofelectron-hole pairs Under the drive of the concentrationdifference the photoelectrons in the Si diffused into thegraphene until the equilibrium state was established Astructure similar to a PN junction was formed between thetwomaterials When the bias was varied from 0 V to -3 V thepeak modulation of the time-domain signal peak achieved51 at optical pump power with 420 mW When the biaschanged from 0 V to -4 V the modulation amplitude depthbecame 83 Based on this structure Ran Jiang et al used a

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

8 Research

Table 2 The comparison of optically tuned modulation properties based on different materials

Work Description Year Frequency Modulation depth RefLibon et al Multiple quantum well 1999 02-1 THz 40 [34]Deng L et al InSb gratings 2013 15 THz 4670 [42]Peter W et al Graphene on Si (GOS) 2012 02-2 THz 99 [56]Wen Q Y et al Graphene on Ge 2014 025-1 THz 94 [57]Cao Y et al MoS2 2016 05- 15THz 96 [62]Fan ZY et al WS2 2017 025- 2 THz 99 [61]Hyung K Yet al C60 2014 05- 15THz 98 [43]He T et al AlClPc 2015 0-25 THz 99 [45]Shi Z et al Si Nanotip 2017 025-1 THz gt90 [44]

compact integration will be the future direction of opticallytuned THz modulator

22 Electrically Tuned THz Modulator The other route formodulating THz wave is electrically tuned THz modulatorUsing a mixed type-Itype-II GaAsAlAs multiple-quantum-well sample Libon et al have demonstrated firstly anelectrically controllable modulator in 2000 [94] Electricalmodulation is to control the concentration of electrons inthe substrate materials or structures by applying bias voltagewhich modulate the amplitude of the incident THz wave[94]The transmission bandwidth andmodulation depth andspeed are limited by dielectric constant loss and responsetime of the materials Consequently to achieve high-speedband-pass modulation or broadband filter it is necessaryto find high-speed low-loss THz functional materials anddesign novel structures

Similar to the main idea of optically tuned THz mod-ulator charge injection can also modulate the THz waveIn the past decade 2D electron gas (2DEG) in HEMTshas been demonstrated to modulate THz wave effectivelyHEMT is a field effect transistor that provides a 2DEGquantum well at the heterojunction interface of a highlydoped semiconductor (typically AlGaAs) and an originalundoped semiconductor (GaAs) In 2000 Kersting presenteda GaAsAlGaAs heterostructure device which powered upthe phase modulation of THz signals [94] This modulatorcontained five parabolic quantum wells (PQWs) THz wavemodulation can be realized by stimulating electrically low-energy electron to high-energy states Figure 7(1)(b) showsthe power spectrum of the differential modulation signaland the power spectrum of the incident pulse It led to thedevelopment of THz electronics chip due to a reduction ofthe device dimensions Nevertheless it cannot operate at lowtemperatures To allow THz modulator to be operated atroom temperature Kleine-Ostmann et al presented room-temperature THz wave modulator by using a device based ona gated 2DEG [66] This modulator mainly included a 2DEGwhose density could be controlled by the gate voltage Thismajor breakthrough illustrated that 2DEG in semiconductorscould be used to control THz wave effectively [95] Similarlyit has also been used for THz emission [96] THz detection[97] etc Other 2DEG systems can be also used to realizethe modulation for example a THz modulator based on

2DEGof a GaNAlGaN heterostructure was demonstrated byZhou (Figure 7(3))Themodulator had amaximum intensitymodulation depth of 93 and a 3dB operating bandwidth of400 kHz It required only a low driving voltage amplitude of2V under 87 K This active plasma-based THz modulatormay provide a promising solution in THz technology fieldsfor the metamaterial THz modulator [67]

221Modulator Based on Semiconductors andMetamaterialsIn addition metamaterials are also a hot spot in THzmodula-tor [98] Kebin Fan et al demonstrated a metamaterial activedevice which consists of gold split-ring resonators (SRRs) onGaAs thin film grown on Si substrate [8] It has achieved50 modulation depth and 100 kHz modulation speedSchottkyn-doped GaAs devices had been developed toincrease greatly the modulation speed In further studies themodulation speed had reached 10 MHz however the modu-lation depth of this modulator was only 30 [99]The single-particle nonresonant absorption mechanism described inDrude model has been used in the related studies As seenfrom experiments mentioned above Drude model has notyet been effective enough for the theory of metamaterialmodulator [100]

222 Modulator Based on Graphene To further improve themodulation depth and modulation speed of THz modulatordifferent active medium as substrate has been tested Therefractive index of those materials can usually be written inthe form of a complex refractive index 119899 = 119899119903 + 119894119899119894 where 119899119903is the real part of the refractive index and 119899119894 the imaginarypart of the refractive index When the voltage is adjustedthe real and imaginary parts are usually changed at the sametime By using those changes different electric modulatorscan be developed For example electroabsorption modulatoris based on imaginary part changing When the real partof the refractive index changed with applied voltage thetransmission phase of THz wave changed Therefore forthe electroabsorption modulator based on 2D material abetter modulation effect can be obtained by changing therefractive index through applied voltage In fact electromod-ulators based on 2D materials mainly focused on grapheneGraphene has many particular properties such as adjustablethin layer conductivity and long mean free path due to itsunique conical energy band structure [101ndash103] Its linear

Research 9

1

(a)

THz wave

THz wave

undoped

undoped

SI GaAs

GaAs

GaAsSi

Cr 5 nmCap 10 nmDoped Layer 50 nm

Spacer 7 nm2 DEG

Buffer 1m

Substrate 500 m

2

3

(a)

(a)

(b)

(b)

0

minus4

minus8

minus12

minus16

minus20

minus24

mod

ulat

ion

(pA

)

(a)

(b)

(b)

0 5 10 15 20

time t (ps)

+50 V+20 V+10 V-05 V-10 V-20 V-50 V-100 V

15

10

5

0

5 0 minus5

minus4 minus3 minus2 minus1 0

minus10gate voltage (V)

Gate voltage (V)

peak

-to-p

eak

ampl

itude

(pA

)

D

W

G A

Split gap

E

H

E

H

10

08

06

04

02

00

06

04

05

02

03

01

00

Tran

smiss

ion

Tran

smitt

ance

05 10 15 20

Frequency (THz)0 V1 V2 V4 V8 V16 V

VG

DS

2DEG Sapphire substrate

SI-GaAs

87K20K50K77K

100K120K150K170K

200K230K300K

Ｈ = 14times1018 ＝Ｇ-3

Figure 7 Electrically tuned THz modulator based on semiconductors and metamaterials Panel (1) (a) The structure of the THz modulator(b) Gate voltage dependence of the modulation signal (upper) Peak-to-peak amplitude of the modulator signal vs gate voltage (nether)Reproduced from [66] Panel (2) (a) Geometry and dimensions of the THz metamaterial switchmodulator (b) Frequency-dependenttransmitted intensity of THz radiation Reproduced from [8] Panel (3) (a) Three-dimensional schematic illustration of the plasmon-basedTHz modulator (b) Dependence of transmission on gate voltage at different temperatures Reproduced from [67]

10 Research

(a) (b)

(a)

(c)

(b)

1

2

3

(b)

(a)

Vg

Terahertzbeam

Terahertzbeam

ℎ]

Vg1 Vg2

EF2gtEF1 (Vg2gtVg1)EF1 (Vg1)

||2

10

08

06

04

02

00

Pow

er re

flect

ance

570 590 610 630

Frequency (GHz)

graphene

V

V = 0

２ rarr 1

２ rarr 0

６ = 0

top graphene layer bottom graphene layer

qV

10

08

06

04

02 07

06

0500

Tran

smitt

ance

Tran

smitt

ance

560

580

600

620

640

660

570 575 580 585

Frequency (GHz)Vg = 50 VVg = 0 V

Figure 8 Electrically tuned THz modulator based on graphene Panel (1) (a) Schematic of the proof-of-concept graphene THz modulator(b) Schematic of the THz modulator (c) Schematic of the device and electric field distribution when the substrate thickness and plasmonicresonance are matched to an odd multiple of a quarter wavelength of the THz wave Reproduced from [68] Panel (2) (a) Schematic of theTHz modulator (b) Normalized reflectance (RR (VCNP)) Reproduced from [69] Panel (3) (a) Schematic of the device and electric fielddistribution (b) Operation mechanism of the self-gated graphene pair Reproduced from [70]

dispersion relationship between energy and crystal momen-tum makes graphene superior to other semiconductor mate-rials

S Rodriguez utilized graphene to modulate THz wave(Figure 8(1)) The modulation depth and modulation speed

could reach 15 and 20 kHz respectively [68] Meanwhilethe same research group presented a reflective THz modu-lator as shown in Figure 8(2) The silver film on the backof the reflective THz modulator acted as both gate andmirror which improved the modulation depth with 64

Research 11

Table 3 The comparison of electrically tuned modulation properties based on different substrate materials

Work Description Year Frequency Modulation depth RefHuang Y D Plasmonic 2016 - 93 [67]Zhou G VO2 2017 03-10THz gt50 [100]Sensale-R B Graphene 2012 10THz 100 [106]Liang G Graphene 2015 3THz 94-100 [71]Huang Z Monolayer Graphene 2018 1-7THz gt85 [72]Chikhi N Liquid crystals 2018 1-3THz gt90 [107]Kaya E Flexible Substrate 2018 02-15THz 100 [74]

and expanded bandwidth ranging 057-063 THz comparedto their early works However its insertion loss and mod-ulation speed only reach 2 dB and 4 kHz respectively[69] The essence of those works is to change the Fermilevel of graphene by adjusting applied voltage The follow-ing graphene THz modulators with different structures arealso based on this method To improve modulation depthmultiple layers of graphene are a choice [102] As shown inFigure 8(3) a THz electroabsorption modulator based ontwo-layer graphene with periodic micron-belt pattern waspresented The THz wave was vertically illustrated on thesurface of graphene which enhanced the absorption of lightbecause of the plasma effect This modulator could operateat frequencies up to tens of THz and the modulation depthcould even reach 100 [103]

Usually the modulation depth of most modulators basedon monolayer graphene can only reach 80 [69] Howevermonolayer graphene combined with other structures ordevices can improve the modulation depth For exampleG Liang puts monolayer graphene on top of quantumcascade laser (QCL) to demonstrate modulation depth of100 (Figure 9(1)) and modulation speed of 110 MHz [71]The high modulation depth was a consequence of a stronginteraction between the graphene and THz wave whereasthe high modulation speed can be improved by reducingthe dimension of device It has recently been demonstratedthat a THz modulator can realize full modulation with awide modulation bandwidth and fast modulation speedUsing monolayer graphene Huang proposed an adjustablecomplementary ring resonator (Figure 9(2)) It was worthmentioning that in the range of 1-2 THz and 3-7 THz threesignificant resonant peaks can be modulated The maximummodulation depth reaches 988 at 747 THz [72]

223 Modulator Based on Flexible Substrate Flexible sub-strate was used to fabricate optical modulator with manymerits [104 105] as described in Section 213 For electricmodulator flexible substrate plays also an important role Forexample Kocabas et al presented a graphene metamaterialTHz modulator based on a flexible substrate (Figure 10(1))which consists of two large-area graphene electrodes trans-ferred onto THz transparent substrates with ionic liquidelectrolytes between them The bias can change electrostaticdoping of graphene which affects its optical properties Inthis case the maximum modulation depth reaches only 50at 01-14 THz working at a bias of 3V The modulation speed

was slow [73] Virtually thismodulator showed excellent flex-ibility which has not been deformed after being flexed manytimes Modulator based on flexible substrate can also realizea high modulation depth For example E Kaya et al havedemonstrated that polyvinyl chloride (PVC) and polyethy-lene (PE) were selected as flexible substrates (Figure 10(2))A multilayer graphene modulator was fabricated on thoseflexible substrates by chemical vapor deposition This devicecould fully modulate THz wave with a frequency range of02-15 THz at a low voltage of 34V [74] The comparison ofelectrically tuned modulation properties based on differentsubstrate materials is shown in Table 3

224 Summary The modulation properties of differentmaterials reported in literatures are given in Table 2Graphene can achieve modulation depth close to 100 andthe modulation speed in the order of kHz At present thereare few reports on flexible THz modulators since organicfunctional materials and flexible substrates cannot fully becompatible with micronanofabrication processes Based onreviewing the electric modulator it implied that electricmodulation device needs to miniaturize the structure to lowinsertion loss and to improve modulation speed which willbe the future direction

23 Photoelectric Hybrid Tuned THz Modulator Sections 21and 22 described the modulation methods using electricityor light separately If two methods can be combined togetherbetter modulation effects may be obtained The photoelectrichybrid modulation method is a method of synthesizing elec-tric and optical modulation Its basic principle is using pumplight excited carriers in a base semiconductor material In themeantime the motion direction of the carrier is adjusted byan external bias [108] For example an optoelectric hybridmodulator was exhibited by Q Li et al [75] Its structurewas shown in Figure 12 Si substrate was pumped with acontinuous laser of 532 nm to generate a large number ofelectron-hole pairs Under the drive of the concentrationdifference the photoelectrons in the Si diffused into thegraphene until the equilibrium state was established Astructure similar to a PN junction was formed between thetwomaterials When the bias was varied from 0 V to -3 V thepeak modulation of the time-domain signal peak achieved51 at optical pump power with 420 mW When the biaschanged from 0 V to -4 V the modulation amplitude depthbecame 83 Based on this structure Ran Jiang et al used a

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

Research 9

1

(a)

THz wave

THz wave

undoped

undoped

SI GaAs

GaAs

GaAsSi

Cr 5 nmCap 10 nmDoped Layer 50 nm

Spacer 7 nm2 DEG

Buffer 1m

Substrate 500 m

2

3

(a)

(a)

(b)

(b)

0

minus4

minus8

minus12

minus16

minus20

minus24

mod

ulat

ion

(pA

)

(a)

(b)

(b)

0 5 10 15 20

time t (ps)

+50 V+20 V+10 V-05 V-10 V-20 V-50 V-100 V

15

10

5

0

5 0 minus5

minus4 minus3 minus2 minus1 0

minus10gate voltage (V)

Gate voltage (V)

peak

-to-p

eak

ampl

itude

(pA

)

D

W

G A

Split gap

E

H

E

H

10

08

06

04

02

00

06

04

05

02

03

01

00

Tran

smiss

ion

Tran

smitt

ance

05 10 15 20

Frequency (THz)0 V1 V2 V4 V8 V16 V

VG

DS

2DEG Sapphire substrate

SI-GaAs

87K20K50K77K

100K120K150K170K

200K230K300K

Ｈ = 14times1018 ＝Ｇ-3

Figure 7 Electrically tuned THz modulator based on semiconductors and metamaterials Panel (1) (a) The structure of the THz modulator(b) Gate voltage dependence of the modulation signal (upper) Peak-to-peak amplitude of the modulator signal vs gate voltage (nether)Reproduced from [66] Panel (2) (a) Geometry and dimensions of the THz metamaterial switchmodulator (b) Frequency-dependenttransmitted intensity of THz radiation Reproduced from [8] Panel (3) (a) Three-dimensional schematic illustration of the plasmon-basedTHz modulator (b) Dependence of transmission on gate voltage at different temperatures Reproduced from [67]

10 Research

(a) (b)

(a)

(c)

(b)

1

2

3

(b)

(a)

Vg

Terahertzbeam

Terahertzbeam

ℎ]

Vg1 Vg2

EF2gtEF1 (Vg2gtVg1)EF1 (Vg1)

||2

10

08

06

04

02

00

Pow

er re

flect

ance

570 590 610 630

Frequency (GHz)

graphene

V

V = 0

２ rarr 1

２ rarr 0

６ = 0

top graphene layer bottom graphene layer

qV

10

08

06

04

02 07

06

0500

Tran

smitt

ance

Tran

smitt

ance

560

580

600

620

640

660

570 575 580 585

Frequency (GHz)Vg = 50 VVg = 0 V

Figure 8 Electrically tuned THz modulator based on graphene Panel (1) (a) Schematic of the proof-of-concept graphene THz modulator(b) Schematic of the THz modulator (c) Schematic of the device and electric field distribution when the substrate thickness and plasmonicresonance are matched to an odd multiple of a quarter wavelength of the THz wave Reproduced from [68] Panel (2) (a) Schematic of theTHz modulator (b) Normalized reflectance (RR (VCNP)) Reproduced from [69] Panel (3) (a) Schematic of the device and electric fielddistribution (b) Operation mechanism of the self-gated graphene pair Reproduced from [70]

dispersion relationship between energy and crystal momen-tum makes graphene superior to other semiconductor mate-rials

S Rodriguez utilized graphene to modulate THz wave(Figure 8(1)) The modulation depth and modulation speed

could reach 15 and 20 kHz respectively [68] Meanwhilethe same research group presented a reflective THz modu-lator as shown in Figure 8(2) The silver film on the backof the reflective THz modulator acted as both gate andmirror which improved the modulation depth with 64

Research 11

Table 3 The comparison of electrically tuned modulation properties based on different substrate materials

Work Description Year Frequency Modulation depth RefHuang Y D Plasmonic 2016 - 93 [67]Zhou G VO2 2017 03-10THz gt50 [100]Sensale-R B Graphene 2012 10THz 100 [106]Liang G Graphene 2015 3THz 94-100 [71]Huang Z Monolayer Graphene 2018 1-7THz gt85 [72]Chikhi N Liquid crystals 2018 1-3THz gt90 [107]Kaya E Flexible Substrate 2018 02-15THz 100 [74]

and expanded bandwidth ranging 057-063 THz comparedto their early works However its insertion loss and mod-ulation speed only reach 2 dB and 4 kHz respectively[69] The essence of those works is to change the Fermilevel of graphene by adjusting applied voltage The follow-ing graphene THz modulators with different structures arealso based on this method To improve modulation depthmultiple layers of graphene are a choice [102] As shown inFigure 8(3) a THz electroabsorption modulator based ontwo-layer graphene with periodic micron-belt pattern waspresented The THz wave was vertically illustrated on thesurface of graphene which enhanced the absorption of lightbecause of the plasma effect This modulator could operateat frequencies up to tens of THz and the modulation depthcould even reach 100 [103]

Usually the modulation depth of most modulators basedon monolayer graphene can only reach 80 [69] Howevermonolayer graphene combined with other structures ordevices can improve the modulation depth For exampleG Liang puts monolayer graphene on top of quantumcascade laser (QCL) to demonstrate modulation depth of100 (Figure 9(1)) and modulation speed of 110 MHz [71]The high modulation depth was a consequence of a stronginteraction between the graphene and THz wave whereasthe high modulation speed can be improved by reducingthe dimension of device It has recently been demonstratedthat a THz modulator can realize full modulation with awide modulation bandwidth and fast modulation speedUsing monolayer graphene Huang proposed an adjustablecomplementary ring resonator (Figure 9(2)) It was worthmentioning that in the range of 1-2 THz and 3-7 THz threesignificant resonant peaks can be modulated The maximummodulation depth reaches 988 at 747 THz [72]

223 Modulator Based on Flexible Substrate Flexible sub-strate was used to fabricate optical modulator with manymerits [104 105] as described in Section 213 For electricmodulator flexible substrate plays also an important role Forexample Kocabas et al presented a graphene metamaterialTHz modulator based on a flexible substrate (Figure 10(1))which consists of two large-area graphene electrodes trans-ferred onto THz transparent substrates with ionic liquidelectrolytes between them The bias can change electrostaticdoping of graphene which affects its optical properties Inthis case the maximum modulation depth reaches only 50at 01-14 THz working at a bias of 3V The modulation speed

was slow [73] Virtually thismodulator showed excellent flex-ibility which has not been deformed after being flexed manytimes Modulator based on flexible substrate can also realizea high modulation depth For example E Kaya et al havedemonstrated that polyvinyl chloride (PVC) and polyethy-lene (PE) were selected as flexible substrates (Figure 10(2))A multilayer graphene modulator was fabricated on thoseflexible substrates by chemical vapor deposition This devicecould fully modulate THz wave with a frequency range of02-15 THz at a low voltage of 34V [74] The comparison ofelectrically tuned modulation properties based on differentsubstrate materials is shown in Table 3

224 Summary The modulation properties of differentmaterials reported in literatures are given in Table 2Graphene can achieve modulation depth close to 100 andthe modulation speed in the order of kHz At present thereare few reports on flexible THz modulators since organicfunctional materials and flexible substrates cannot fully becompatible with micronanofabrication processes Based onreviewing the electric modulator it implied that electricmodulation device needs to miniaturize the structure to lowinsertion loss and to improve modulation speed which willbe the future direction

23 Photoelectric Hybrid Tuned THz Modulator Sections 21and 22 described the modulation methods using electricityor light separately If two methods can be combined togetherbetter modulation effects may be obtained The photoelectrichybrid modulation method is a method of synthesizing elec-tric and optical modulation Its basic principle is using pumplight excited carriers in a base semiconductor material In themeantime the motion direction of the carrier is adjusted byan external bias [108] For example an optoelectric hybridmodulator was exhibited by Q Li et al [75] Its structurewas shown in Figure 12 Si substrate was pumped with acontinuous laser of 532 nm to generate a large number ofelectron-hole pairs Under the drive of the concentrationdifference the photoelectrons in the Si diffused into thegraphene until the equilibrium state was established Astructure similar to a PN junction was formed between thetwomaterials When the bias was varied from 0 V to -3 V thepeak modulation of the time-domain signal peak achieved51 at optical pump power with 420 mW When the biaschanged from 0 V to -4 V the modulation amplitude depthbecame 83 Based on this structure Ran Jiang et al used a

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

10 Research

(a) (b)

(a)

(c)

(b)

1

2

3

(b)

(a)

Vg

Terahertzbeam

Terahertzbeam

ℎ]

Vg1 Vg2

EF2gtEF1 (Vg2gtVg1)EF1 (Vg1)

||2

10

08

06

04

02

00

Pow

er re

flect

ance

570 590 610 630

Frequency (GHz)

graphene

V

V = 0

２ rarr 1

２ rarr 0

６ = 0

top graphene layer bottom graphene layer

qV

10

08

06

04

02 07

06

0500

Tran

smitt

ance

Tran

smitt

ance

560

580

600

620

640

660

570 575 580 585

Frequency (GHz)Vg = 50 VVg = 0 V

Figure 8 Electrically tuned THz modulator based on graphene Panel (1) (a) Schematic of the proof-of-concept graphene THz modulator(b) Schematic of the THz modulator (c) Schematic of the device and electric field distribution when the substrate thickness and plasmonicresonance are matched to an odd multiple of a quarter wavelength of the THz wave Reproduced from [68] Panel (2) (a) Schematic of theTHz modulator (b) Normalized reflectance (RR (VCNP)) Reproduced from [69] Panel (3) (a) Schematic of the device and electric fielddistribution (b) Operation mechanism of the self-gated graphene pair Reproduced from [70]

dispersion relationship between energy and crystal momen-tum makes graphene superior to other semiconductor mate-rials

S Rodriguez utilized graphene to modulate THz wave(Figure 8(1)) The modulation depth and modulation speed

could reach 15 and 20 kHz respectively [68] Meanwhilethe same research group presented a reflective THz modu-lator as shown in Figure 8(2) The silver film on the backof the reflective THz modulator acted as both gate andmirror which improved the modulation depth with 64

Research 11

Table 3 The comparison of electrically tuned modulation properties based on different substrate materials

Work Description Year Frequency Modulation depth RefHuang Y D Plasmonic 2016 - 93 [67]Zhou G VO2 2017 03-10THz gt50 [100]Sensale-R B Graphene 2012 10THz 100 [106]Liang G Graphene 2015 3THz 94-100 [71]Huang Z Monolayer Graphene 2018 1-7THz gt85 [72]Chikhi N Liquid crystals 2018 1-3THz gt90 [107]Kaya E Flexible Substrate 2018 02-15THz 100 [74]

and expanded bandwidth ranging 057-063 THz comparedto their early works However its insertion loss and mod-ulation speed only reach 2 dB and 4 kHz respectively[69] The essence of those works is to change the Fermilevel of graphene by adjusting applied voltage The follow-ing graphene THz modulators with different structures arealso based on this method To improve modulation depthmultiple layers of graphene are a choice [102] As shown inFigure 8(3) a THz electroabsorption modulator based ontwo-layer graphene with periodic micron-belt pattern waspresented The THz wave was vertically illustrated on thesurface of graphene which enhanced the absorption of lightbecause of the plasma effect This modulator could operateat frequencies up to tens of THz and the modulation depthcould even reach 100 [103]

Usually the modulation depth of most modulators basedon monolayer graphene can only reach 80 [69] Howevermonolayer graphene combined with other structures ordevices can improve the modulation depth For exampleG Liang puts monolayer graphene on top of quantumcascade laser (QCL) to demonstrate modulation depth of100 (Figure 9(1)) and modulation speed of 110 MHz [71]The high modulation depth was a consequence of a stronginteraction between the graphene and THz wave whereasthe high modulation speed can be improved by reducingthe dimension of device It has recently been demonstratedthat a THz modulator can realize full modulation with awide modulation bandwidth and fast modulation speedUsing monolayer graphene Huang proposed an adjustablecomplementary ring resonator (Figure 9(2)) It was worthmentioning that in the range of 1-2 THz and 3-7 THz threesignificant resonant peaks can be modulated The maximummodulation depth reaches 988 at 747 THz [72]

223 Modulator Based on Flexible Substrate Flexible sub-strate was used to fabricate optical modulator with manymerits [104 105] as described in Section 213 For electricmodulator flexible substrate plays also an important role Forexample Kocabas et al presented a graphene metamaterialTHz modulator based on a flexible substrate (Figure 10(1))which consists of two large-area graphene electrodes trans-ferred onto THz transparent substrates with ionic liquidelectrolytes between them The bias can change electrostaticdoping of graphene which affects its optical properties Inthis case the maximum modulation depth reaches only 50at 01-14 THz working at a bias of 3V The modulation speed

was slow [73] Virtually thismodulator showed excellent flex-ibility which has not been deformed after being flexed manytimes Modulator based on flexible substrate can also realizea high modulation depth For example E Kaya et al havedemonstrated that polyvinyl chloride (PVC) and polyethy-lene (PE) were selected as flexible substrates (Figure 10(2))A multilayer graphene modulator was fabricated on thoseflexible substrates by chemical vapor deposition This devicecould fully modulate THz wave with a frequency range of02-15 THz at a low voltage of 34V [74] The comparison ofelectrically tuned modulation properties based on differentsubstrate materials is shown in Table 3

224 Summary The modulation properties of differentmaterials reported in literatures are given in Table 2Graphene can achieve modulation depth close to 100 andthe modulation speed in the order of kHz At present thereare few reports on flexible THz modulators since organicfunctional materials and flexible substrates cannot fully becompatible with micronanofabrication processes Based onreviewing the electric modulator it implied that electricmodulation device needs to miniaturize the structure to lowinsertion loss and to improve modulation speed which willbe the future direction

23 Photoelectric Hybrid Tuned THz Modulator Sections 21and 22 described the modulation methods using electricityor light separately If two methods can be combined togetherbetter modulation effects may be obtained The photoelectrichybrid modulation method is a method of synthesizing elec-tric and optical modulation Its basic principle is using pumplight excited carriers in a base semiconductor material In themeantime the motion direction of the carrier is adjusted byan external bias [108] For example an optoelectric hybridmodulator was exhibited by Q Li et al [75] Its structurewas shown in Figure 12 Si substrate was pumped with acontinuous laser of 532 nm to generate a large number ofelectron-hole pairs Under the drive of the concentrationdifference the photoelectrons in the Si diffused into thegraphene until the equilibrium state was established Astructure similar to a PN junction was formed between thetwomaterials When the bias was varied from 0 V to -3 V thepeak modulation of the time-domain signal peak achieved51 at optical pump power with 420 mW When the biaschanged from 0 V to -4 V the modulation amplitude depthbecame 83 Based on this structure Ran Jiang et al used a

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

Research 11

Table 3 The comparison of electrically tuned modulation properties based on different substrate materials

Work Description Year Frequency Modulation depth RefHuang Y D Plasmonic 2016 - 93 [67]Zhou G VO2 2017 03-10THz gt50 [100]Sensale-R B Graphene 2012 10THz 100 [106]Liang G Graphene 2015 3THz 94-100 [71]Huang Z Monolayer Graphene 2018 1-7THz gt85 [72]Chikhi N Liquid crystals 2018 1-3THz gt90 [107]Kaya E Flexible Substrate 2018 02-15THz 100 [74]

and expanded bandwidth ranging 057-063 THz comparedto their early works However its insertion loss and mod-ulation speed only reach 2 dB and 4 kHz respectively[69] The essence of those works is to change the Fermilevel of graphene by adjusting applied voltage The follow-ing graphene THz modulators with different structures arealso based on this method To improve modulation depthmultiple layers of graphene are a choice [102] As shown inFigure 8(3) a THz electroabsorption modulator based ontwo-layer graphene with periodic micron-belt pattern waspresented The THz wave was vertically illustrated on thesurface of graphene which enhanced the absorption of lightbecause of the plasma effect This modulator could operateat frequencies up to tens of THz and the modulation depthcould even reach 100 [103]

Usually the modulation depth of most modulators basedon monolayer graphene can only reach 80 [69] Howevermonolayer graphene combined with other structures ordevices can improve the modulation depth For exampleG Liang puts monolayer graphene on top of quantumcascade laser (QCL) to demonstrate modulation depth of100 (Figure 9(1)) and modulation speed of 110 MHz [71]The high modulation depth was a consequence of a stronginteraction between the graphene and THz wave whereasthe high modulation speed can be improved by reducingthe dimension of device It has recently been demonstratedthat a THz modulator can realize full modulation with awide modulation bandwidth and fast modulation speedUsing monolayer graphene Huang proposed an adjustablecomplementary ring resonator (Figure 9(2)) It was worthmentioning that in the range of 1-2 THz and 3-7 THz threesignificant resonant peaks can be modulated The maximummodulation depth reaches 988 at 747 THz [72]

223 Modulator Based on Flexible Substrate Flexible sub-strate was used to fabricate optical modulator with manymerits [104 105] as described in Section 213 For electricmodulator flexible substrate plays also an important role Forexample Kocabas et al presented a graphene metamaterialTHz modulator based on a flexible substrate (Figure 10(1))which consists of two large-area graphene electrodes trans-ferred onto THz transparent substrates with ionic liquidelectrolytes between them The bias can change electrostaticdoping of graphene which affects its optical properties Inthis case the maximum modulation depth reaches only 50at 01-14 THz working at a bias of 3V The modulation speed

was slow [73] Virtually thismodulator showed excellent flex-ibility which has not been deformed after being flexed manytimes Modulator based on flexible substrate can also realizea high modulation depth For example E Kaya et al havedemonstrated that polyvinyl chloride (PVC) and polyethy-lene (PE) were selected as flexible substrates (Figure 10(2))A multilayer graphene modulator was fabricated on thoseflexible substrates by chemical vapor deposition This devicecould fully modulate THz wave with a frequency range of02-15 THz at a low voltage of 34V [74] The comparison ofelectrically tuned modulation properties based on differentsubstrate materials is shown in Table 3

224 Summary The modulation properties of differentmaterials reported in literatures are given in Table 2Graphene can achieve modulation depth close to 100 andthe modulation speed in the order of kHz At present thereare few reports on flexible THz modulators since organicfunctional materials and flexible substrates cannot fully becompatible with micronanofabrication processes Based onreviewing the electric modulator it implied that electricmodulation device needs to miniaturize the structure to lowinsertion loss and to improve modulation speed which willbe the future direction

23 Photoelectric Hybrid Tuned THz Modulator Sections 21and 22 described the modulation methods using electricityor light separately If two methods can be combined togetherbetter modulation effects may be obtained The photoelectrichybrid modulation method is a method of synthesizing elec-tric and optical modulation Its basic principle is using pumplight excited carriers in a base semiconductor material In themeantime the motion direction of the carrier is adjusted byan external bias [108] For example an optoelectric hybridmodulator was exhibited by Q Li et al [75] Its structurewas shown in Figure 12 Si substrate was pumped with acontinuous laser of 532 nm to generate a large number ofelectron-hole pairs Under the drive of the concentrationdifference the photoelectrons in the Si diffused into thegraphene until the equilibrium state was established Astructure similar to a PN junction was formed between thetwomaterials When the bias was varied from 0 V to -3 V thepeak modulation of the time-domain signal peak achieved51 at optical pump power with 420 mW When the biaschanged from 0 V to -4 V the modulation amplitude depthbecame 83 Based on this structure Ran Jiang et al used a

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

12 Research

(a)

1

2

(a)

(b)

(b)

Px

Py ２Ｉ ２Ｃ

y(H) z(k)

x(E)

1009080706050403020100

1-T

T0

1 2 3 4 5 6 7Frequency (THz)

= 04eV

16

12

8

4

0

Volta

ge (V

)

00 05 10 15 20 25 30

Current (A)

10

08

06

04

02

00

lowast2

lowast5

20K50K70K

90K102K

8

4

0

8

4

04

2

0

Inte

nsity

(au

)

29 30 31 32 33 34 35 36

Frequency (THz)

292A 20K

282A 20K

272A 20K

43210minus1minus2minus3minus4

Figure 9 Electrically tuned THz modulator based on graphene Panel (1) (a) Overview of the quantum cascade laser integrated graphenemodulator (b) Light-current-voltage (LIV) characteristics of the THz CCG QCL at different heat sink temperatures (Upper) Laser spectraas a function of pump current I (Nether) Reproduced from [71] Panel (2) (a) Right schematic of the light interacting with graphene CSRRLeft schematic of the graphene CSRR (b) Extinction spectrum in transmission of the graphene CSRRs (upper) Distributions of normalizedz-component of electric field Ez at 138THz 433THz and 5THz respectively (nether) Reproduced from [72]

SiHfO2 material added between graphene and Si substrateto realize a device that had a modulation effect underboth biasing directions [76] However it is more difficultto implement a hybrid modulation method since there arecompletely different responses when applying positive ornegative biases Quan Li et al realized an active modulation

at low voltage (sim1V) by a photoelectric hybrid modulatorwith a two-dimensional material on a structured Si substrate[77]

As a newmodulation method photoelectric hybrid tunedTHzmodulator has not drawnmuch attention However thisextreme high sensitive method is in need urgently

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

Research 13

(a)(b)

(a)

(b)

V

1

2

Incident

THz pulse Refle

cted

Transmitted

Grapheneelectrodes

100

80

60

40

20

0

Tran

smiss

ion

()

05 10 15

Frequency (THz)

-1V-2V

-3V

gold electrode

multilayergraphene

1

01

Am

plitu

de (a

u)

03 08 13 18

Frequency (THz)

Figure 10 Electrically tuned THzmodulator based on flexible substrate Panel (1) (a) Schematic representation of the graphene supercapacitorused as a broadband THzmodulator (b) Spectrumof the transmitted THz signal obtained after Fourier transformation of the recorded signaland normalization Reproduced from [73] Panel (2) (a) THz setup and MLG structure (inset) consisting of MLG sandwiched between host(PVC or PE) electrolyte and gold electrode (b) Corresponding frequency domain amplitudes Reproduced from [74]

(a)

(b)

Electrode

Graphene

Si

(c)(d)

SiHf2

09

07

05

03

01

Tran

smiss

ion

peak

minus8 minus6 minus4 minus2 0 2 4 6

Voltage (V)

0 mW140 mW

280 mW420 mW

Figure 11 Photoelectric hybrid tuned THz modulator (a) Illustration of the GSTD sample The double-layer graphene on the Si substrate wasphotoexcited with green light and biased with voltage Vg (b) Gate voltage-dependent normalized time-domain transmission peaks of thedouble-layer graphene on Si (c) Schematic drawings of working mechanism for the GSS structure (d) Monolayer graphene deposited onthe SRRs with continuous wave laser light excitation and bias voltage Vg Reproduced from [75ndash77]

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

14 Research

(a) (b)

1

80m80m

65m

9m

070

068

066

064

062

060

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

070

068

066

064

062

060

058

Freq

uenc

y (T

Hz)

Freq

uenc

y (T

Hz)

0 5 10

strain ()

(a) (b)

(c) (d)

0 5 10

strain ()

0 5 10

strain ()0 5 10

strain ()

I1 I2

I2 I2

1st cycle

2nd cycle 5th cycle

stretchingreleasing

080

078

076

074

072

070

stretchingreleasing

stretchingreleasing

stretchingreleasing

32m

2

Figure 12 Self-deformation tuned THz modulator Panel (1) THz modulator structure diagram (a) and modulation performance diagram(b) based on metamaterial Reproduced from [78] Panel (2) (a) Schematic of the film metamaterial unit structure with double-layer USRRsfor THz wave phase shifter (b) Overview of the developed flexible film metamaterial device Reproduced from [79]

24 Mechanically Tuned THz Modulator THz modulatorson flexible substrates have been introduced in Sections 213and 223 These devices can also be applied to complexnonplanar surfaces such as communications fibers aircraftand radar surfaces [75 109] Alternatively the modulation ofTHz wave was achieved by preparing metal metastructureson the surface and changing space between them [110] Forexample in 2013 Li et al presented a flexible and tunableTHz metamaterial (Figure 11(1)) The tensile deformation ofsubstrate caused change of the pitch which in turn affectedthe transmission of THz waves Thus this THz device canbe used to detect the deformation of different objects Whenthe deformation appeared multiple times there were metal

fatigue phenomena which affected the stability This methodcan also achieve phase modulation For example an ultrathinTHz wave phase shifter was described by ZL Han [79] Eachmetamaterial unit consists of double-layered structure Thedistance change between two layers affects phase shift whichcaused phase delay This phase shifter has high transmissionwith a coefficient of 91 Compared to traditional THz wavephase shifters this ultrathin flexible phase shifter has bettersignal transmission or reflection (Figure 11(2)) It could beintegrated with other systems to improve device adjustabilityThese findings paved the way for flexible THz electronicsand contributed to the development of THz technology[111]

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

Research 15

(a)

(b)

30

20

10

5

1

Fiel

d en

hanc

emen

t09

08

07

06

05

04

03

Tran

smiss

ion

02 04 06 08 10

Frequency (THz)

LC Dipole

300 K330 K339 K

345 K350 K

Figure 13 Thermally tuned THz modulator (a) Resonant field enhancement as a function of position (b) Temperature-dependent THztransmission spectra of SRRs on VO2 Reproduced from [80]

Nowadays flexible electronic devices have graduallybecome a research hotspot with the development of wearableelectronic devices flexible displays and health monitoringdevices One type of modulator affects the intensity variationof the THz wave by its self-deforming [112] Unfortunatelythere are no reports on THz modulation devices with highstability in the deformed state [113] Therefore studying aTHz modulator with stable modulation performance underflexible deformation is a problem to be solved

25 Thermally Tuned THz Modulator In many transitionmetal oxides the modulation of THz wave is achieved byexternal stimuli including temperature light electric fieldmechanical strain or magnetic field [80 114 115] Tempera-ture changes can affect the mobility and lifetime of free carri-ers in thosematerials Vanadiumdioxide (VO2 ) film is ametaloxide with an insulator-metal phase transition propertywhich can be converted from insulating state (monoclinicstructure) to metallic state (tetragonal structure) under heat[116 117] It leads to reversiblemutation in physical properties[118 119] Both theory and experiment have exhibited thatVO2 film has high transmittance in the insulating phaseand opposite property in the metal phase [120] ThereforeVO2 is a good thin film phase change material suitable forthe THz modulator (Figure 13) which consists of periodicmetastructure on the surface of VO2 then THz wave can bemodulated by controlling the temperature [121]

26 Magnetically Tuned THz Modulator Magnetic-tunedterahertz modulator was based on the magnetooptical effectunder an external magnetic field For example magnetizedplasma 2D photonic crystal THz wave modulator was pre-sented byWen (Figure 14(1)) [81]The resonant frequency canbe tuned with the insertion loss of 03 dB The modulationspeed was as high as 4 GHz Thus this modulator has

the potential for THz wireless broadband communicationsystem

A THz wave modulator based on Fe3O4-nanoparticleswas reported recently (Figure 14(2)) [82] This modulatorconsists of a magnetic fluid and metamaterial structure Themodulator confirmed a 34 modulation depth It is worthmentioning that metamaterials cause a 33 GHz frequencyshift at low magnetic field This modulator will have manypotential applications in THz filtering modulation andsensing

Although magnetically tuned THz has more applicationsunfortunately they are poorly controllable in the currentstudy

27 MEMS Tuned THz Modulator Microelectromechani-cal systems (MEMS) are a high-tech frontier discipline inmodern information technology [122ndash124] The continuousimprovement of MEMS technology provides superiority forTHzmodulator For exampleH Tao et al [83] achievedmod-ulation of the resonant intensity in the THz band (05 THz)based on thermal drive technology in 2009 Q Bai [84] et aldemonstrated a planar semiconductor metamaterial deviceutilizingMEMS-based slow-light tunable effects which couldtune over a wide frequency range in the THz band Ozbeyand Aktas [85] presented THz metamaterial device that usedmagnetically driven resonant frequency (Figure 15(1)) ZHan demonstrated a reconfigurable metamaterial structurefabricated byMEMS technique [125]The resonant frequencycan be tuned by using the voltage to control the height ofthe center metal ring (Figure 15(2)) The modulation rangewas relatively narrow which could only achieve modulationbetween 045 THz and 065 THz [86] According to themerit ofMEMS technology theMEMS tunedTHzmodulatorwith high modulation efficiency could achieve extensiveapplications in high-power field

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

16 Research

1

2

(a)

y

xz

SiInSb

TETE

(b)

0000 05

02 04170

172

174

176

178

180

182

06 08 10 12 14 16

10Frequency (THz)

Frequency (THz)

Refr

activ

e Ind

ex

15

0 mT52 mT99 mT

147 mT194 mT

20

10n

20n

30n

40n

Tran

smiss

ion

Inte

nsity

(au

)

(a)

MetamaterialFerrofluids

Cuvette

(b)

0 mT54 mT99 mT

147mT194mT

Figure 14 Magnetically tuned THz modulator Panel (1) (a) The structure model of THz wave modulator based on magnetized plasmaPC (b) The real part (solid) and image part (dotted) of neffof InSb in the THz regime with the dependence of the external magnetic fieldReproduced from [81] Panel (2) (a) Schematic of the modulator structure (b) Refractive index of the ferrofluid at different magnetic fieldintensities Reproduced from [82]

28 THz Modulator Based on Coding and ProgrammableMetamaterials The unique electromagnetic properties ofelectromagnetic metamaterials result in rapid evolving Forexample in 2014 Giovampaola and Engheta proposed amethod of constructing metamaterials through spatiallymixed ldquodigital metamaterial bitsrdquo [126] This new conceptmetamaterial can be used in THz modulator At the sameyear Tie Jun Cui et al proposed the concept of codedmetamaterials [87] They abandoned the traditional methodof effective medium theory and designed the encoded meta-materials of 1-bit code sequences which were used to flexiblymodulate THz waves As shown from Figure 16 the ldquo0rdquoand ldquo1rdquo elements represent the ideal magnetic and electricalconductors respectively The phase difference after reflectionis 180∘ According to the traditional phase-array-antennatheory the scattering pattern on the metasurfaces under the

coding sequence can be calculated to design different codingsequences Take the coded metasurfaces composed of NtimesNsquare grids with dimension D as an example [87 127]

119889119894119903 (120579 120593) = 4120587 1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162int21205870int12058720

1003816100381610038161003816119891 (120579 120593)10038161003816100381610038162 sin 120579119889120579119889120593 (6)

where 120579 and 120593 are the elevation and azimuth anglesrespectively and 119891(120579 120593) is the pattern function of a latticeThat is to say different encodings produce different modu-lation effects When the normal incident wave illuminatesthe metasurfaces two and four reflected beams are formed(Figure 16) which illustrate that metasurfaces can modulatethe THz wave A 2-bit random coding arrangement was usedto obtain the expected low-scattering pattern which achievedwide-band diffuse reflection of terahertz waves [88] Shuo

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

Research 17

(b)

(a)

(b) (c)

1

2

(a)

xb

p

q

m

w

d

l

yx

z

y

E

Hk

E

H

k

330 m78 m

100 m

6 m

8 m

8 m

OFF-State

Au 024 m

Air Gap 2 m

Au 024 m

SIDE VIEW

minus100

minus300

minus500

minus700

Phas

e(d

eg)

02 03 04 05 06 07 08

Frequency f (THz)

1

05

0

Tran

smiss

ion

Coe

ffici

entT

ON-state (g=0 m)OFF-state (g=2 m)

02 03 04 05 06 07 08

Frequency f (THz)ON-state (g=0 m)OFF-state (g=2 m)

LL

4L

L = 100 m

6 mR 14 m

E

HHHHHHHH

k

LL

4L

L =Si2 024 m

Si2 02 m

Figure 15 MEMS tuned THz modulator Panel (1) Schematic illustration of unit cell Reproduced from [83ndash85] Panel (2) (a) MEMSreconfigurable SRR design (b) Tunable THz switch performance phase (upper) and transmission amplitude (nether) Reproduced from[86]

Liu et al proposed the concept of anisotropically encodedmetamaterials in 2016 [128] Their function depends onthe polarization direction of the incident wave The beamsplitter used in metasurface can separate the orthogonalpolarization modulated terahertz signals which can be usedto increase the transmission rate in ultra-high-speed wirelesscommunication in the future Coding and programmablemetamaterials a new approach to study and design fromthe information perspective provide us with great flexibilityin controlling the radiation of the electromagnetic wave inboth amplitude and phase [129]The research results illustratethat programmable metamaterials are a possible route toimplement more functional THz modulators in the future

3 Conclusions and Outlook

We have extensively reviewed modulators operating in THzfrequency range and summarized the principles currentstatus and advantages and disadvantages of various methods

At present electrical and optical modulation THz deviceshave achieved exciting results in terms of modulation depthand modulation speed Compared with electrical modula-tion optical modulation has deeper modulation depth fastermodulation speed and easier modulation method Althoughthe photoelectric hybrid modulation can be more flexiblethe modulation method is obviously complicated Othermethods provide new attempts for THzmodulation howeverthe extension and simple implementation in the frequencyrange need further study In terms of material selection 2Dmaterials and flexible substrates have become the focus ofrecent experimental research due to their huge advantagesProgrammable metamaterials dynamically control the phaseresponses for each element It can be considered as a possibleroute for the future realization of THz modulators with farmore functionalities

THz modulator research is a dynamic fast-growing andchallenging field An ideal modulation device needs a higherdegree of miniaturization lower insertion loss and higher

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

18 Research

(b)(a)

(c)

(d)

0

-30

(a)

(b)

12

12 14 16 18 20Frequency (THz)

SimExp

With metasurface

Inc

Gold

x

y

z

LOR and AZ5214PolyimideSilicon wafer

WO metasurface10

10

08

08

06

Refle

ctio

n

040200

1

2

360300240180120600

minus60minus120minus180

Phas

e (de

gree

)

7 8 9 10 11 12 13 14

Frequency (GHz)

Phase difference

ldquo1rdquo element

ldquo0rdquo element

Metala

ℎt

wMetal

Substrate

Figure 16 THz modulator based on coding and programmable metamaterials Panel (1) The 1-bit digital metasurfaces and codingmetasurfaces (a) Two types of elements ldquo0rdquo and ldquo1rdquo (b) Different phase responses of two elements in a range of frequencies (c) Differentencodings produce different modulation effects (d) Simulation calculation result Reproduced from [87] Panel (2) The fabrication andmeasurement results of the coding metasurfaces (a) The fabrication process for the coding metasurfaces (b) The measured and simulatedbackward scattering coefficients of the 2-bit coding metasurfaces Reproduced from [88]

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

Research 19

modulation speed which will be the core of building a THzcommunication system More explorations are required forvarious actual devices to get an excellent modulation effect

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this article

Authorsrsquo Contributions

Zhaoxin Geng conceived and outlined structure of thisreview Zhaoxin Geng Jian Liu and Hongda Chen revisedthe manuscript Zhengtai Ma wrote the manuscript ZhiyuanFan helped to revise the figures

Acknowledgments

The work was financially supported by The National Key Ramp D Plan of China (2016YFB0402700 2017YFB0405400)TheNational Natural Science Foundation of China (6177417561674146 and 61875140)TheNational Natural Science Foun-dation of China (Key Program 61634006)TheOpened Fundof the State Key Laboratory of Integrated Optoelectronics(No IOSKL2017KF12) and the Key Program of Natural Sci-ence Foundation of Beijing (4181001)The Leading Project ofYouth Academic Team Minzu University of China (Sensorand Microsystem 317201929) and Young and Middle-AgedTalents Training Program of State Ethnic Affairs Commis-sion

References

[1] J W Fleming ldquoHigh-resolution submillimeter-wave fourier-transform spectrometry of gasesrdquo IEEE Transactions onMicrowaveTheory and Techniques vol 22 no 12 pp 1023ndash10251974

[2] I S Osborne ldquoApplied physics filling the THz gaprdquo Science vol320 no 5881 pp 1262bndash1262b 2008

[3] G P Williams ldquoFilling the THz gap - high power sources andapplicationsrdquo Reports on Progress in Physics vol 69 no 2 pp301ndash326 2006

[4] D E Spence P N Kean andW Sibbett ldquo60-fsec pulse genera-tion from a self-mode-lockedTisapphire laserrdquoOptics Expresssvol 16 no 1 pp 42ndash44 1991

[5] P H Siegel ldquoTerahertz technologyrdquo IEEE Transactions onMicrowave Theory and Techniques vol 50 no 3 pp 910ndash9282002

[6] M Tonouchi ldquoCutting-edge terahertz technologyrdquoNature Pho-tonics vol 1 no 2 pp 97ndash105 2007

[7] M Nagel P Haring Bolivar M Brucherseifer H Kurz ABosserhoff and R Buttner ldquoIntegrated THz technology forlabel-free genetic diagnosticsrdquo Applied Physics Letters vol 80no 1 pp 154ndash156 2002

[8] H-T ChenW J Padilla J M O Zide A C Gossard A J Tay-lor and R D Averitt ldquoActive terahertz metamaterial devicesrdquoNature vol 444 no 7119 pp 597ndash600 2006

[9] T Kleine-Ostmann and T Nagatsuma ldquoA review on terahertzcommunications researchrdquo Journal of Infrared Millimeter andTerahertz Waves vol 32 no 2 pp 143ndash171 2011

[10] J Federici and LMoeller ldquoReview of terahertz and subterahertzwireless communicationsrdquo Journal of Applied Physics vol 107no 11 article 111101 2010

[11] H-J Song and T Nagatsuma ldquoPresent and future of terahertzcommunicationsrdquo IEEE Transactions on Terahertz Science andTechnology vol 1 no 1 pp 256ndash263 2011

[12] H Hoshina S Nakajima M Yamashita C Otani and NMiyoshi ldquoTerahertz imaging diagnostics of the cancer tissueswith chemometrics techniquerdquo Applied Physics Letters vol 90no 4 2006

[13] P H Siegel ldquoTerahertz technology in biology and medicinerdquoIEEE Transactions onMicrowaveTheory and Techniques vol 52no 10 pp 2438ndash2447 2004

[14] B Ferguson S Wang D Gray D Abbott and X-C ZhangldquoIdentification of biological tissue using chirped probe THzimagingrdquoMicroelectronics Journal vol 33 no 12 pp 1043ndash10512002

[15] H-B Liu H Zhong N Karpowicz Y Chen and X-C ZhangldquoTerahertz spectroscopy and imaging for defense and securityapplicationsrdquo Proceedings of the IEEE vol 95 no 8 pp 1514ndash1527 2007

[16] D Zimdars ldquoFiber-pigtailed terahertz time domain spec-troscopy instrumentation for package inspection and securityimagingrdquo in Proceedings of the Terahertz for Military andSecurity Applications pp 108ndash116 USA April 2003

[17] J Li R Yu X Hao A Zheng and X Yang ldquoCoherent laser-induced optical behaviors in three-coupled-quantum wells andtheir application to terahertz signal detectionrdquo Optics Commu-nications vol 282 no 22 pp 4384ndash4389 2009

[18] F RutzMKoch S KhareMMonekeHRichter andU EwertldquoTerahertz quality control of polymeric productsrdquo InternationalJournal of Infrared andMillimeter Waves vol 27 no 4 pp 547ndash556 2006

[19] S Nishizawa Y Suzuki T IwamotoMW Takeda andM TanildquoTerahertz time-domain spectroscopy (THz-TDS) approach tothe quality control on pharmaceutical productsrdquo in Proceedingsof the 35th International Conference on Infrared Millimeter andTerahertz Waves IRMMW-THz 2010 Italy September 2010

[20] T Crowe ldquoMultiplier technology for terahertz applicationsrdquo inProceedings of the IEEE Sixth International Conference pp 58ndash61 1998

[21] M J Fitch and R Osiander ldquoTerahertz waves for communica-tions and sensingrdquo Johns Hopkins APL Technical Digest vol 25no 4 pp 348ndash355 2004

[22] S Jine R Jian and L Wenxin ldquoProgress of terahertz incommunication technologyrdquo Infrared and Laser Engineeringvol 35 no 3 pp 342ndash347 2006

[23] T Kosugi M Tokumitsu T Enoki M Muraguchi A Hirataand T Nagatsuma ldquo120-GHz TxRx chipset for 10-Gbits wire-less applications using 01 120583m-gate InP HEMTsrdquo in Proceedingsof the IEEE Compound Semiconductor Integrated Circuit Sympo-sium 2004 IEEE CSIC Symposium pp 171ndash174 USA October2004

[24] A Hirata T Kosugi H Takahashi et al ldquo120-GHz-bandmillimeter-wave photonic wireless link for 10-Gbs data trans-missionrdquo IEEE Transactions on Microwave Theory and Tech-niques vol 54 no 5 pp 1937ndash1942 2006

[25] R Yamaguchi A Hirata T Kosugi et al ldquo10-Gbits MMICwireless link exceeding 800 metersrdquo in Proceedings of the 2008IEEE Radio and Wireless Symposium RWS pp 695ndash698 USAJanuary 2008

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

20 Research

[26] T NagatsumaandAHirata ldquo10-Gbitswireless link technologyusing the 120-GHz bandrdquo NTT Technical Review vol 2 no 11pp 58ndash62 2004

[27] S Cherry ldquoEdholmrsquos law of bandwidthrdquo IEEE Spectrum vol 41no 7 pp 58ndash60 2003

[28] J Yao N Chi P Yang et al ldquoStudy and outlook of terahertzcommunication technologyrdquo Chinese Journal of Lasers vol 36no 9 pp 2213ndash2233 2009

[29] J Zhou D R Chowdhury R Zhao et al ldquoTerahertz chiralmetamaterials with giant and dynamically tunable optical activ-ityrdquo Physical Review B CondensedMatter andMaterials Physicsvol 86 no 3 Article ID 035448 2012

[30] D K Kim and D S Citrin ldquoFrequency and amplitude mod-ulation in terahertz-sideband generation in quantum wellsrdquoApplied Physics Letters vol 94 no 2 Article ID 021105 2009

[31] J Li ldquoTerahertz modulator using photonic crystalsrdquo OpticsCommunications vol 269 no 1 pp 98ndash101 2007

[32] O Paul C Imhof B Lagel et al ldquoPolarization-independentactive metamaterial for high-frequency terahertz modulationrdquoOptics Express vol 17 no 2 pp 819ndash827 2009

[33] N Necker I Libon M Hempel J Feldmann M Koch and PDawson ldquoAn optically-driven THz-modulatorrdquo in Proceedingsof the Technical Digest Summaries of papers presented at theConference on Lasers and Electro-Optics Postconference EditionCLEO rsquo99 Conference on Lasers and Electro-Optics pp 370-371Baltimore Md USA 1999

[34] I H Libon S Baumgartner M Hempel et al ldquoAn opticallycontrollable terahertz filterrdquo Applied Physics Letters vol 76 no20 pp 2821ndash2823 2000

[35] Z Xie X Wang J Ye et al ldquoSpatial Terahertz ModulatorrdquoScientific Reports vol 3 no 1 article 3347 2013

[36] W L Chan H Chen A J Taylor I Brener M J Cich and DM Mittleman ldquoA spatial light modulator for terahertz beamsrdquoApplied Physics Letters vol 94 no 21 Article ID 213511 2009

[37] R A Shelby D R Smith and S Schultz ldquoExperimentalverification of a negative index of refractionrdquo Science vol 292no 5514 pp 77ndash79 2001

[38] M Tanaka F Miyamaru M Hangyo T Tanaka M Akazawaand E Sano ldquoEffect of a thin dielectric layer on terahertz trans-mission characteristics for metal hole arraysrdquo Optics Expresssvol 30 no 10 pp 1210ndash1212 2005

[39] R Ulbricht E Hendry J Shan T F Heinz andM Bonn ldquoCar-rier dynamics in semiconductors studied with time-resolvedterahertz spectroscopyrdquo Reviews of Modern Physics vol 83 no2 pp 543ndash586 2011

[40] J-S Li and J-Q Yao ldquoNovel optical controllable terahertz waveswitchrdquoOptics Communications vol 281 no 23 pp 5697ndash57002008

[41] L Fekete F Kadlec P Kuzel and H Nemec ldquoUltrafast opto-terahertz photonic crystal modulatorrdquo Optics Expresss vol 32no 6 pp 680ndash682 2007

[42] L Deng J Teng H Liu et al ldquoDirect optical tuning of theterahertz plasmonic response of insb subwavelength gratingsrdquoAdvanced Optical Materials vol 1 no 2 pp 128ndash132 2013

[43] H K Yoo Y Yoon K Lee et al ldquoHighly efficient terahertzwavemodulators by photo-excitation of organicssilicon bilayersrdquoApplied Physics Letters vol 105 no 1 Article ID 011115 2014

[44] Z Shi X Cao Q Wen et al ldquoTerahertz modulators based onsilicon nanotip arrayrdquo Advanced Optical Materials vol 6 no 2Article ID 1700620 2018

[45] T He B Zhang and J Shen ldquoHigh-efficiency THz modulatorbased on phthalocyanine-compound organic filmsrsquordquo AppliedPhysics Letters vol 106 no 5 2015

[46] H K Yoo H Lee K Lee et al ldquoConditions for optimalefficiency of PCBM-based terahertzmodulatorsrdquoAIPAdvancesvol 7 no 10 Article ID 105008 2017

[47] L Jiu-Sheng L Shao-He and Z Le ldquoTerahertz modulatorusing 4-N N-dimethylamino-41015840-N1015840-methyl-stilbazolium tosy-late (DAST)Si hybrid structurerdquo IEEE Photonics Journal vol10 no 2 pp 1ndash6 2018

[48] J B Prendry A J Holden W J Stewart and I YoungsldquoExtremely low frequency plasmons in metallic mesostruc-turesrdquo Physical Review Letters vol 76 no 25 pp 4773ndash47761996

[49] J B Pendry A J Holden D J Robbins andW J Stewart ldquoMag-netism from conductors and enhanced nonlinear phenomenardquoIEEE Transactions onMicrowaveTheory and Techniques vol 47no 11 pp 2075ndash2084 1999

[50] W J Padilla A J Taylor CHighstreteM Lee andRDAverittldquoDynamical electric and magnetic metamaterial response atterahertz frequenciesrdquo Physical Review Letters vol 96 no 102006

[51] L Cong Y K Srivastava H Zhang X Zhang J Han andR Singh ldquoAll-optical active THz metasurfaces for ultrafastpolarization switching and dynamic beam splittingrdquo LightScience amp Applications vol 7 no 1 2018

[52] X Chen S Ghosh Q Xu et al ldquoActive control of polarization-dependent near-field coupling in hybrid metasurfacesrdquo AppliedPhysics Letters vol 113 no 6 Article ID 061111 2018

[53] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[54] P-Y Chen and A Alu ldquoTerahertz metamaterial devices basedon graphene nanostructuresrdquo IEEE Transactions on TerahertzScience and Technology vol 3 no 6 pp 748ndash756 2013

[55] P R Wallace ldquoThe band theory of graphiterdquo Physical Review AAtomic Molecular and Optical Physics vol 71 no 9 pp 622ndash634 1947

[56] J C Slonczewski and P R Weiss ldquoBand structure of graphiterdquoPhysical Review A Atomic Molecular and Optical Physics vol109 no 2 pp 272ndash279 1958

[57] M Liu X Yin E Ulin-Avila et al ldquoA graphene-based broad-band optical modulatorrdquo Nature vol 474 no 7349 pp 64ndash672011

[58] PWeis J LGarcia-PomarMHoh B ReinhardA BrodyanskiandM Rahm ldquoSpectrally wide-band terahertzwave modulatorbased on optically tuned graphenerdquoACSNano vol 6 no 10 pp9118ndash9124 2012

[59] Q Wen W Tian Q Mao et al ldquoGraphene based all-opticalspatial terahertz modulatorrdquo Scientific Reports vol 4 no 1article 7409 2015

[60] M Fu X Wang S Wang et al ldquoEfficient terahertz modulatorbased on photoexcited graphenerdquoOptical Materials vol 66 pp381ndash385 2017

[61] Z Fan Z Geng X Lv et al ldquoOptical controlled terahertzmodulator based on tungsten disulfide nanosheetrdquo ScientificReports vol 7 no 1 2017

[62] Y Cao S Gan Z Geng et al ldquoOptically tuned terahertz mod-ulator based on annealed multilayer MoS2rdquo Scientific Reportsvol 6 no 1 Article ID 22899 2016

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

Research 21

[63] D-S Yang T Jiang and X-A Cheng ldquoOptically con-trolled terahertzmodulator by liquid-exfoliatedmultilayerWS2nanosheetsrdquo Optics Express vol 25 no 14 pp 16364ndash163772017

[64] D-Y Khang H Jiang Y Huang and J A Rogers ldquoA stretchableform of single-crystal silicon for high-performance electronicson rubber substratesrdquo Science vol 311 no 5758 pp 208ndash2122006

[65] K Fan X Zhao J Zhang et al ldquoOptically tunable terahertzmetamaterials on highly flexible substratesrdquo IEEE Transactionson Terahertz Science and Technology vol 3 no 6 pp 702ndash7082013

[66] T Kleine-Ostmann P Dawson K Pierz G Hein and MKoch ldquoRoom-temperature operation of an electrically driventerahertz modulatorrdquo Applied Physics Letters vol 84 no 18 pp3555ndash3557 2004

[67] Y D Huang Y Yu H Qin et al ldquoPlasmonic terahertz mod-ulator based on a grating-coupled two-dimensional electronsystemrdquo Applied Physics Letters vol 109 no 20 Article ID201110 2016

[68] B Sensale-Rodriguez R Yan M M Kelly et al ldquoBroadbandgraphene terahertz modulators enabled by intraband transi-tionsrdquo Nature Communications vol 3 article 780 2012

[69] B Sensale-Rodriguez R Yan S Rafique et al ldquoExtraordinarycontrol of terahertz beam reflectance in graphene electro-absorption modulatorsrdquo Nano Letters vol 12 no 9 pp 4518ndash4522 2012

[70] B Sensale-Rodriguez R Yan M Zhu D Jena L Liu and HGrace Xing ldquoEfficient terahertz electro-absorptionmodulationemploying graphene plasmonic structuresrdquo Applied PhysicsLetters vol 101 no 26 Article ID 261115 2012

[71] G Liang X Hu X Yu et al ldquoIntegrated terahertz graphenemodulator with 100 modulation depthrdquo ACS Photonics vol2 no 11 pp 1559ndash1566 2015

[72] Z Huang Q Han C Ji J Wang and Y Jiang ldquoBroadbandterahertz modulator based on graphene metamaterialsrdquo AIPAdvances vol 8 no 3 Article ID 035304 2018

[73] N Kakenov O Balci E O Polat H Altan and C KocabasldquoBroadband terahertz modulators using self-gated graphenecapacitorsrdquo Journal of the Optical Society of America B OpticalPhysics vol 32 no 9 pp 1861ndash1866 2015

[74] E Kaya N Kakenov H Altan C Kocabas and O EsenturkldquoMultilayer graphene broadband terahertz modulators withflexible substraterdquo Journal of Infrared Millimeter and TerahertzWaves vol 39 no 5 pp 483ndash491 2018

[75] Q Li Z Tian X Zhang et al ldquoActive graphenendashsilicon hybriddiode for terahertz wavesrdquo Nature Communications vol 6 no1 article 7082 2015

[76] R Jiang Z Han W Sun X Du Z Wu and H Jung ldquoFerro-electricmodulation of terahertzwaveswith grapheneultrathin-SiHfO2Si structuresrdquo Applied Physics Letters vol 107 no 15Article ID 151105 2015

[77] Q Li Z Tian X Zhang et al ldquoDual control of active graphene-silicon hybrid metamaterial devicesrdquo Carbon vol 90 pp 146ndash153 2015

[78] J Li C M Shah W Withayachumnankul et al ldquoMechanicallytunable terahertz metamaterialsrdquo Applied Physics Letters vol102 no 12 Article ID 121101 2013

[79] Z Han S Ohno Y Tokizane et al ldquoThin terahertz-wave phaseshifter by flexible film metamaterial with high transmissionrdquoOptics Express vol 25 no 25 pp 31186ndash31196 2017

[80] M Liu H Y Hwang H Tao et al ldquoTerahertz-field-inducedinsulator-to-metal transition in vanadium dioxide metamate-rialrdquo Nature vol 487 no 7407 pp 345ndash348 2012

[81] W Zhou H-M Chen K Ji and Y Zhuang ldquoVerticallymagnetic-controlled THz modulator based on 2-Dmagnetizedplasma photonic crystalrdquo Photonics and Nanostructures - Fun-damentals and Applications vol 23 pp 28ndash35 2017

[82] X Liu L Xiong X Yu S He B Zhang and J ShenldquoMagnetically controlled terahertz modulator based on Fe3O4nanoparticle ferrofluidsrdquo Journal of Physics D Applied Physicsvol 51 no 10 Article ID 105003 2018

[83] H Tao A C Strikwerda K Fan W J Padilla X Zhang andRDAveritt ldquoReconfigurable terahertzmetamaterialsrdquoPhysicalReview Letters vol 103 no 14 2009

[84] J Chen ldquoTunable slow light in semiconductormetamaterial in abroad terahertz regimerdquo Journal of Applied Physics vol 107 no9 Article ID 093104 2010

[85] B Ozbey and O Aktas ldquoContinuously tunable terahertz meta-material employing magnetically actuated cantileversrdquo OpticsExpress vol 19 no 7 pp 5741ndash5752 2011

[86] Z Han K Kohno H Fujita K Hirakawa and H ToshiyoshildquoTunable terahertz filter and modulator based on electrostaticMEMS reconfigurable SRR arrayrdquo IEEE Journal of SelectedTopics in Quantum Electronics vol 21 no 4 pp 114ndash122 2015

[87] T J Cui M Q Qi X Wan J Zhao and Q Cheng ldquoCodingmetamaterials digital metamaterials and programmable meta-materialsrdquo Light Science amp Applications vol 3 no 10 pp e218ndashe218 2014

[88] L Gao Q Cheng J Yang et al ldquoBroadband diffusion of tera-hertz waves by multi-bit coding metasurfacesrdquo Light Science ampApplications vol 4 no 9 pp e324ndashe324 2015

[89] K F Mak C Lee J Hone J Shan and T F Heinz ldquoAtomicallythin MoS2 a new direct-gap semiconductorrdquo Physical ReviewLetters vol 105 no 13 2010

[90] X Liu B Zhang G Wang W Wang H Ji and J Shen ldquoActiveterahertz wave modulator based on molybdenum disulfiderdquoOptical Materials vol 73 pp 718ndash722 2017

[91] Y J Yoo H Y Zheng Y J Kim et al ldquoFlexible and elasticmetamaterial absorber for low frequency based on small-sizeunit cellrdquo Applied Physics Letters vol 105 no 4 Article ID041902 2014

[92] N R Han Z C Chen C S Lim B Ng and M H HonldquoBroadband multi-layer terahertz metamaterials fabricationand characterization on flexible substratesrdquoOptics Express vol19 no 8 pp 6990ndash6998 2011

[93] S Lee S Kim T Kim et al ldquoMetamaterials reversiblystretchable and tunable terahertz metamaterials with wrinkledlayoutsrdquo Advanced Materials vol 24 no 26 pp 3438-34382012

[94] R Kersting G Strasser and K Unterrainer ldquoTerahertz phasemodulatorrdquo IEEE Electronics Letters vol 36 no 13 pp 1156ndash1158 2000

[95] S Rout D Shrekenhamer S Sonkusale and W PadillaldquoEmbedded HEMTmetamaterial composite devices for activeterahertz modulationrdquo in Proceedings of the 23rd Annual Meet-ing of the IEEE pp 437-438 2010

[96] J Mateos S Perez D Pardo et al ldquoTerahertz emission andnoise spectra in HEMTsrdquo in Proceedings of the AIP Conferencevol 800 pp 423ndash430 2005

[97] T Watanabe S B Tombet Y Tanimoto et al ldquoUltrahighsensitive plasmonic terahertz detector based on an asymmetric

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017

22 Research

dual-grating gate HEMT structurerdquo Solid-State Electronics vol78 pp 109ndash114 2012

[98] H-T Chen W J Padilla M J Cich A K Azad R D Averittand A J Taylor ldquoA metamaterial solid-state terahertz phasemodulatorrdquo Nature Photonics vol 3 no 3 pp 148ndash151 2009

[99] D Shrekenhamer S Rout A C Strikwerda et al ldquoHigh speedterahertz modulation from metamaterials with embedded highelectron mobility transistorsrdquo Optics Express vol 19 no 10 pp9968ndash9975 2011

[100] G Zhou P Dai J Wu et al ldquoBroadband and high modulation-depthTHzmodulator using lowbias controlledVO2-integratedmetasurfacerdquo Optics Express vol 25 no 15 pp 17322ndash173282017

[101] K F Mak M Y Sfeir Y Wu C H Lui J A Misewich and T FHeinz ldquoMeasurement of the optical conductivity of graphenerdquoPhysical Review Letters vol 101 no 19 Article ID 196405 2008

[102] Y Zhang Y-W Tan H L Stormer and P Kim ldquoExperimentalobservation of the quantum Hall effect and Berryrsquos phase ingraphenerdquoNature vol 438 no 7065 pp 201ndash204 2005

[103] A K Geim ldquoGraphene status and prospectsrdquo Science vol 324no 5934 pp 1530ndash1534 2009

[104] P D CunninghamN N Valdes F A Vallejo et al ldquoBroadbandterahertz characterization of the refractive index and absorp-tion of some important polymeric and organic electro-opticmaterialsrdquo Journal of Applied Physics vol 109 no 4 Article ID043505 2011

[105] Y G Rabobason G P Rigas S Swaisaenyakorn et al ldquoDesignof flexible passive antenna array on kapton substraterdquo Progressin Electromagnetics Research C vol 63 pp 105ndash117 2016

[106] Y Wu C La-O-Vorakiat X Qiu et al ldquoGraphene terahertzmodulators by ionic liquid gatingrdquo Advanced Materials vol 27no 11 pp 1874ndash1879 2015

[107] N Chikhi M Lisitskiy G Papari V Tkachenko and AAndreone ldquoA hybrid tunable THz metadevice using a highbirefringence liquid crystalrdquo Scientific Reports vol 6 no 1 2016

[108] I Maeng S Lim S J Chae Y H Lee H Choi and J-H SonldquoGate-controlled nonlinear conductivity of Dirac fermion ingraphene field-effect transistors measured by terahertz time-domain spectroscopyrdquo Nano Letters vol 12 no 2 pp 551ndash5552012

[109] J Li C M Shah W Withayachumnankul et al ldquoFlexibleterahertz metamaterials for dual-axis strain sensingrdquo OpticsExpresss vol 38 no 12 pp 2104ndash2106 2013

[110] Z-G Wang Y-F Chen P-J Li et al ldquoFlexible graphene-basedelectroluminescent devicesrdquo ACS Nano vol 5 no 9 pp 7149ndash7154 2011

[111] L ZhangM Zhang andH Liang ldquoRealization of full control ofa terahertz wave using flexible metasurfacesrdquo Advanced OpticalMaterials vol 5 no 24 Article ID 1700486 2017

[112] S Aksu M Huang A Artar et al ldquoFlexible plasmonics onunconventional and nonplanar substratesrdquoAdvancedMaterialsvol 23 no 38 pp 4422ndash4430 2011

[113] J Liu P Li Y Chen et al ldquoFlexible terahertz modulator basedon coplanar-gate graphene field-effect transistor structurerdquoOptics Letters vol 41 no 4 article 816 2016

[114] P Limelette A Georges D Jerome P Wzietek P Metcalf andJ M Honig ldquoUniversality and critical behavior at the Motttransitionrdquo Science vol 302 no 5642 pp 89ndash92 2003

[115] J Wang J B Neaton H Zheng et al ldquoEpitaxial BiFeO3multiferroic thin film heterostructuresrdquo Science vol 299 no5613 pp 1719ndash1722 2003

[116] A Zylbersztejn and N F Mott ldquoMetal-insulator transition invanadium dioxiderdquo Physical Review B Condensed Matter andMaterials Physics vol 11 no 11 pp 4383ndash4395 1975

[117] E E Chain ldquoOptical properties of vanadium dioxide andvanadium pentoxide thin filmsrdquo Applied Optics vol 30 no 19pp 2782ndash2787 1991

[118] H Kim Y W Lee B Kim et al ldquoMonoclinic and correlatedmetal phase inVO2 as evidence of themott transition coherentphonon analysisrdquo Physical Review Letters vol 97 no 26 2006

[119] R Lopez L A Boatner T E Haynes R F Haglund Jr and LC Feldman ldquoSwitchable reflectivity on silicon froma compositeVO2-SiO2 protecting layerrdquo Applied Physics Letters vol 85 no8 pp 1410ndash1412 2004

[120] T Ben-Messaoud G Landry J P Gariepy B RamamoorthyP V Ashrit and A Hache ldquoHigh contrast optical switching invanadium dioxide thin filmsrdquo Optics Communications vol 281no 24 pp 6024ndash6027 2008

[121] A Cavalleri C Toth C W Siders J Squier F Raski and JKieffer ldquoFemtosecond structural dynamics in VO 2 during anultrafast solid-solid phase transitionrdquo Physical Review Lettersvol 87 no 23 Article ID 237401 2001

[122] J Zhu ldquoDevelopment and applications of MEMS technologyrdquoSemiconductor Science and Technology vol 28 no 1 pp 29ndash322003

[123] M GadelhakMEMS Design and Fabrication Crc Press 2006[124] M Gadelhak MEMS Introduction and Fundamentals Crc

Press 2005[125] Z Han K Kohno H Fujita K Hirakawa and H Toshiyoshi

ldquoMEMS reconfigurable metamaterial for terahertz switchablefilter and modulatorrdquo Optics Express vol 22 no 18 pp 21326ndash21339 2014

[126] C Della Giovampaola and N Engheta ldquoDigital metamaterialsrdquoNature Materials vol 13 no 12 pp 1115ndash1121 2014

[127] S Liu and T J Cui ldquoConcepts working principles and appli-cations of coding and programmable metamaterialsrdquoAdvancedOptical Materials vol 5 no 22 Article ID 1700624 2017

[128] S Liu T J Cui Q Xu et al ldquoAnisotropic coding metamate-rials and their powerful manipulation of differently polarizedterahertz wavesrdquo Light Science amp Applications vol 5 no 5 ppe16076ndashe16076 2016

[129] T J Cui S Liu and L Zhang ldquoInformation metamaterials andmetasurfacesrdquo Journal of Materials Chemistry C vol 5 no 15pp 3644ndash3668 2017