Compact room temperature terahertz imaging:

towards on–chip integration

Gintaras Valušis, Linas Minkevičius, Irmantas

Kašalynas, Rimvydas Venckevičius, Dalius Seliuta,

Vincas Tamošiūnas

Department of Optoelectronics,

Center for Physical Sciences and Technology,

A. Goštauto 11, LT–01108, Vilnius, Lithuania

Bogdan Voisiat, Gediminas Račiukaitis

Department of Laser Technologies,

Center for Physical Sciences and Technology,

Savanorių Ave. 231, LT-02300, Vilnius, Lithuania

Abstract— On-chip integration of secondary diffractive optics

and bow-tie-shaped InGaAs-based terahertz detectors is

presented. Zone plates were produced directly on the bottom

surface of a 500 µm-thick semi-insulating InP substrate

employing direct laser write technique. Integration of the bow-tie

detector and the zone plate allows to enhance detection more

than one order of magnitude at 0.76 THz. Good correlation

between experimental data and 3D finite-difference time-domain

simulation results is found. It is confirmed that observed

detection enhancement is caused mainly by the focusing

performance of the zone plate.

Keywords – compact optics, laser ablation, terahertz imaging,

InGaAs, bow-tie sensors

I. INTRODUCTION

A large variety of possible applications of terahertz (THz)

radiation employing its ability to propagate through non-

conducting materials and packaging/closing substances

requires new solutions in making convenient-in-usage designs

for compact room temperature imaging and spectroscopy

[1,2]. A particular attention needs to be paid not only to the

development of compact THz emitters, sensitive detectors

exhibiting low-noise properties and higher dynamic range, but

also to the fabrication of compact optics elements and their

possible integration with such active components. A given

approach should prevent undesirable misalignment issues in

real-life application environment operating THz imaging

equipment.

In this communication, we discuss possible compact optics

solutions via zone plates and cross-shaped apertures of the

resonant THz filter and describe a new route in the

miniaturisation of the THz image recording setup by the on-

chip integration of diffractive optics elements with InGaAs-

based bow-tie THz detectors [3]. The fabrication sequence

consists of two technological steps – processing of the InGaAs

detector array on the top surface of the InP wafer, while the

bottom of the InP wafer was metallized and then structured

technologically to form flat-optics focusing elements.

On-chip integration of such focusing elements with detectors

facilitated an increase of the detected signal by an order of

magnitude at 0.76 THz and showed a track for the design of

entirely solid-state based compact THz image recording

systems free of optical alignment issues.

II. ON-CHIP DESIGNS AND FABRICATION

Points of departure in designs were the following parameters:

working frequency of Fresnel zone plates (Fig. 1 (a)) and

cross-shaped apertures of resonant THz filtering (Fig. 1(b))

was tuned to 0.76 THz, which corresponds to the one of the

most stable lines of the optically pumped molecular THz laser

FIRL-100 (Edinburgh Instruments Ltd); the focal length of

0.5 mm was determined by the InP substrate thickness.

The on-chip design of compact optics and the THz sensor is

depicted in Fig. 2. The laser direct writing system [4] was used

to open six Fresnel zones of the zone plate in a 200 nm-thick

gold layer, deposited on the bottom surface of the 500-µm-

thick semi-insulating InP:Fe (001) substrate.

In the focal point of the Fresnel zone, the InGaAs bow-tie

sensor ((Fig. 2(c)) was produced on another surface of the

wafer with molecular-beam-epitaxy-grown layer of 520-nm-

thick In0.47Ga0.53As and a monolayer of InAs; the geometry of

the diodes is similar to that described in Ref. 3.

Electrical contacts were made by evaporation of Ti (20 nm)

and 180 nm of Au, followed by relevant rapid annealing.

Finite-difference time-domain (FDTD) calculations were

applied to model focusing performance of the designed

integrated zone plate. A simulation area was 3×3×1 mm3 XYZ

which includes the detector, substrate segment, zone plate and

several hundred microns of space around. Spatial resolution

was 5 μm. Absorbing boundary conditions were specified so

that at the edges of the simulation area in all directions so that

numerical-only ‘reflections’ of the scattered waves were

suppressed. A transparent multi-frequency plane-wave source

was specified on the top of the simulation space in the z-

direction. It provided a simulated plane-wave source with a

variable angle of incidence.

Simulation results are given in Fig. 2(b). As it easily seen that

978-1-5090-2214-4/16/$31.00 ©2016 IEEE

the incident THz light of 0.76 THz frequency is sharply focused into InGaAs bow-tie diode operating in unbiased conditions – due to non-uniform carrier heating the dc voltage is induced by the contacts of the diodes [5].

III. EXPERIMENTAL RESULTS AND DISCUSSIONS

Results of our simulations confirmed that that the maximum value of the electric field amplitude can be reached under normal incidence of the THz light when the detector is placed in the focus of the zone plate as illustrated in Fig. 2(b). In order to resolve the influences of zone plate focusing performance and the sensitivity of the InGaAs bow-tie sensor itself on its output signal, one needs to vary incident angle. It is shown theoretically [6] that the turn of angle of incidence by 11° is quite sufficient to obtain two distinct maxima in the detector plane – the first corresponds to the shifted focus of the zone plate while the other one – to the electric field enhancement by the bow-tie antenna itself. In more details, even without the detector, several open zones are sufficient to increase the amplitude of the electric field in 5.5 times from a 0.9 mean value without the detector and zone plate. One deserves noting that the bow-tie antenna shape of the detector itself provides a similar order of magnitude enhancement due to the concentration of the electric field near the tip. When both of these effects coupled combined, an enhancement should increase well above order of magnitude is obtained near the tip of the contact [6].

Experimental results are presented in Fig. 3, where profiles of the detected signal at two incident angles α are given. The first one corresponds to normal incidence, α = 0

o; another one,

when α = 17o, denotes tilted incident wave.

Terahertz radiation was delivered by optically pumped molecular THz laser FIRL-100 (Edinburgh Instruments Ltd) adjusted to 0.76 THz line. The power of 3.9 mW was modulated with an optical chopper at 410 Hz frequency. The beam was divided into two parts by a beam splitter (transmittance–reflectance ratio 9:1). The first beam was directed to the reference – pyroelectric detector, whereas the other one was used for focusing and measurement by the main detector.

The signals were registered with lock-in amplifiers. The beam

profile of the laser radiation was recorded in the focus of the

parabolic mirror of 10 cm focal length.

As one can see from Fig. 3, maximum detected signal is

obtained under normal illumination of the detection system

composed of the InGaAs detector and the zone plate.

Deviation from the normal incidence causes approximately

more one order of magnitude signal reduction within all

measured coordinate range.

Much lower signal and relatively weak dependence on the

angle of incidence were determined for a detector without

integrated zone plate on another surface of the wafer. These

observations are in good agreement with the 3D finite-

difference time-domain simulation data, confirming hence that

the observed enhancement of the detected signal can indeed be

attributed to the focusing effect of the zone plate.

To illustrate advantages of combination of the zone plate and

laser-ablated resonant filter, images of 0.76 THz laser beam

focused with conventional zone plate and the zone plate

containing resonant filters were measured using CMOS field-

effect transistors with nanometric gates as THz sensors with a

pixel size of 100×100 μm2.

Fig. 1. a) Layout of the zone plate; b) Combination of the zone plate and laser-ablated resonant filter,

Geometry and the scale: distance between centers of the

closest resonant elements in zones was L = 240 μm, width and length of the cross-shape apertures are M = 40 μm and

K = 200 μm, respectively.

Grey colour represents metal area.

0 1 2 3 4 50.1

1

10

100

Sig

nal (

V)

Coordinate (mm)

Det.+ zones, =0 o Det., =0

o

Det.+ zones, =17 o Det., =17

o

Fig. 3. 0.76 THz beam profiles recorded at different incident radiation

angles measured with combined bow tie detector (Det. + zones) and

with bow tie detector of the same design without integrated zone plate

(Det). Pixel size is 0.1 mm.

Fig. 2. Design of THz bow-tie detector with integrated zone plate on

InP substrate. (a) SEM image of laser-ablated zone plate on bottom of InP substrate. (b) Sketch of the detector with simulated distribution of

the electric field amplitude in InP substrate under illumination of THz

radiation from the back surface of the substrate. Note that the electric field is concentrated into InGaAs placed on front surface of the

substrate. (c) Photo of the InGaAs bow-tie THz detector fabricated on

the front surface of substrate.

One can also notice that ripples around the central maximum

are much weaker for a case of the cross-shaped filters

integrated zone plates; diameter of the focused beam spot is

smaller, and the focusing performance is higher than that of

the conventional zone plate [7].

The presented approach can be extended to compact

spectroscopic THz imaging systems [8].

To summarize, on-chip integrated zone plates and InGaAs

bow-tie-diode THz detectors were designed and fabricated.

Detector signal enhancement of more than one order of

magnitude was obtained due to focusing effect of the zone

plate.

Focusing performance of the terahertz zone plate with

integrated cross shape apertures was investigated at selected

0.76 THz frequency. The THz laser beam shaping with the

zone plate containing laser-ablated filters was found to be

more efficient in comparison to that with the conventional

zone plate.

REFERENCES

[1] P. U. Jepsen, D. G. Cooke, and M. Koch, “Terahertz spectroscopy and

imaging – Modern techniques and applications,” Laser Photon. Rev., vol. 5, pp. 124-166, January 2011.

[2] P. H. Siegel, “Terahertz technology,” IEEE Trans. Microwave Theory Tech., vol. 50, pp. 910-928, March 2002.

[3] L. Minkevičius, V. Tamošiūnas, I. Kašalynas, D. Seliuta, G. Valušis, A. Lisauskas, S. Boppel, H. G. Roskos, and K. Köhler, “Terahertz heterodyne imaging with InGaAs-based bow-tie diodes,” App. Phys. Lett., vol. 99, pp. 131101-1131101-3, September 2011.

[4] B. Voisiat, A. Bičiūnas, I. Kašalynas, and G. Račiukaitis, “Band-pass filters for THz spectral range fabricated by laser ablation,” Appl. Phys. A, vol. 104, pp. 953–958, May 2011.

[5] D. Seliuta, I. Kašalynas, V. Tamošiūnas, S. Balakauskas, Z. Martūnas, S. Ašmontas, G. Valušis, A. Lisauskas, H. G. Roskos and K. Köhler, “Silicon lens-coupled bow-tie InGaAs-based broadband terahertz sensor operating at room temperature,” Electron Lett., vol. 42, pp. 825-827, July 2006.

[6] L. Minkevičius, V. Tamošiūnas, K. Madeikis, B. Voisiat, I. Kašalynas and G. Valušis, “On-chip integration of laser-ablated zone plates for detection enhancement of InGaAs bow-tie terahertz detectors,” Electron. Lett., vol. 50, pp. 1367-1369, September 2014.

[7] L. Minkevičius, K. Madeikis, B. Voisiat, A. Mekys, R. Venckevičius, I. Kašalynas, G. Račiukaitis, G. Valušis, and V. Tamošiūnas, “Focusing performance of terahertz zone plates with integrated cross-shape apertures,” J. Infrared MilliTHz Waves, vol. 35, pp. 699-702, July 2014.

[8] I. Kašalynas, R. Venckevičius, D. Seliuta, I. Grigelionis, and G. Valušis, “InGaAs-based bow-tie diode for spectroscopic terahertz imaging,” J. Appl. Phys., vol. 110, pp. 114505-1- 114505-6, December 2011.

Fig. 4. Simulated and measured spatial profiles of the THz laser beam at

the focal position with conventional (a) and combined (b) zone plates,

respectively. Pixel size is 100×100 µm2. Simulation results are

displayed with dashed lines, measurement data - with solid lines.

Corresponding zone plate design and images of 0.76 THz laser beam focused with the conventional (a) zone plates and the combined zone

plate with THz filters (b) are presented in insets of the figure.