Chapter 14New THz Technologies and Applicationsin Applications in Support of Safety and Security

Ashok Vaseashta

Abstract Recent incidents have prompted changes to the methods employed forsecurity screening at airports and border security check-points. At cargo screeningfacilities and major border check-points, where thousands of containers need to bescreened rapidly, it is a challenging task to effectively screen each container. Asa result, there is an increasing focus on new technologies that can be applied forsecurity screening in a stand-off mode, either to simplify or speed up the screeningprocess, or to provide additional functionality. Terahertz (THz) technology is apromising and emerging technology and has been considered in various forms.Additionally in the battlefield, one of the major threat vectors is improvisedexplosive devices (IEDs) used in different forms such as vehicle borne IEDs(VBIEDs) or strapped to humans at inconspicuous locations. THz imaging systemscan be used to image such threat vectors, since such materials have characteristicTHz spectra. The use of THz illumination of sufficient power levels and fast imagedetection and processing, has shown that non-metallic weaponry can be imagedwhen concealed beneath clothing. Some of the barrier and potential confusingmaterials have smooth spectra with relatively low attenuation. However, use of theTHz frequencies, initially aimed at narrow-band at 830 GHz along with opticalmixing can be used to identify metal and dielectric objects. The ultimate possibilityto identify the chemical compositions of explosive materials and mixed chemicalcompositions needs the wide-band antennas. Potential use of THz imaging in activeand passive imaging systems for detection of chemical and biological agents andremote monitoring of signals is described. Selected applications of THz for stand-off

A. Vaseashta (�)Institute for Advanced Sciences Convergence, NUARI, 13873 Park Center Rd. Suite 500,Herndon, VA 20171, USA

International Clean Water Institute, NUARI, 13873 Park Center Rd. Suite 500,Herndon, VA 20171, USAe-mail: avaseash@norwich.edu

C. Corsi and F. Sizov (eds.), THz and Security Applications, NATO Science for Peaceand Security Series B: Physics and Biophysics, DOI 10.1007/978-94-017-8828-1__14,© Springer ScienceCBusiness Media Dordrecht 2014

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detection are described by using nanomaterials to generate and detect responsesignal, and also to demonstrate that the THz spectra of several common chemicals,explosive compounds, and pharmaceuticals are distinct for ease of identification.

14.1 Introduction – Threat Vectors: Emerging, Persistent,Dual Use and Avant Garde

The current geopolitical landscape is exceedingly complex, dynamic, andunpredictable. Many threat vectors have become highly asymmetric, kinetic,and non-linear. Traditional rules-of-engagement do not apply or have differentinterpretations. Figure 14.1 shows many ways by which adversaries can launchovert/covert attacks, including psychological operations (PSYOPS), Chemical,biological, radiological, nuclear (CBRN), improvised explosive devices (IEDs),

Fig. 14.1 Methods of terrorism including CBRNE, cyber, water, and Bitcoin with current andfuture countermeasure platforms

14 New THz Technologies and Applications in Applications in Support. . . 279

non-traditional agents (NTA), cyber-attacks, and raise funds using non-traditionalmethods (viz. hawala, Bitcoins) to support such operations, along with current andfuturistic countermeasure platforms to defeat terrorism nexus. It is worth noting thatTHz is currently considered as a potential technological platform for many applica-tions. STANDEX1 is one such platform and is jointly considered by many countries.

Notwithstanding the unparalleled level of technological advances, the asym-metric threat from terrorist groups continues to evolve. S&T advances coupledwith universal access to the Internet provides the same means by which stateand non-state sponsored actors develop warfare agents with a certain level ofsophistication. Effective countermeasures also use essentially the same tools, henceunderstanding of transformational emerging sciences, concepts and theories, andtheir potential applications in support of defense and security is exceedingly criticalas effective countermeasures. Numerous technological advances arise from thepotential of nanoscale materials to exhibit unique properties that are attributable totheir reduced dimensions [1]. Furthermore, advances in material synthesis, devicefabrication and characterization have provided the means to study, understand,control, or even manipulate the transitional characteristics between isolated atomsand molecules, and bulk materials. Consequently, various new “materials by design”capable of producing devices and systems with remarkable, tunable, and specificproperties have recently been fabricated. Such advances coupled with informationtechnology, cognitive sciences, biotechnology, artificial intelligence, and geneticsoffer an ecosystem of innovations and potential pathways to counter threat vectorsin ways never imagined possible earlier. A nexus of technological innovations toinclude deployment of systems with enhanced and remote maneuverability, remoteinterrogation and mitigation, enhanced information gathering, and thwarting threatsat the point-of-origin (PO2) is described elsewhere [2].

This report describes use of THz spectrum to detect explosives and harmfulvapors, image hidden metal objects that can serve as weapons, and receive elec-tromagnetic signals from devices emitting such signals from a distance. Examplesof innovative countermeasures using advanced and nano materials exploiting THzregion of the electro-magnetic spectrum to provide various functionalities are alsodiscussed. Tactically, the unsophisticated nature of threats poses significantly greatertechnical challenges in both point and stand-off detection. This threat is growing dueto increased globalization and mobility within society, the explosion in chemicaland biotech expertise and the relative ease with which chemical weapons can beprepared at off-sites, and the means by which pathogens can be covertly engineered,transported and released or dispersed.

Understanding the emerging security challenges (ESC) and nature and potentialof threats may prevent or minimize a potentially catastrophic occurrence. Mostpotential threats have been characterized by a severity of hazard (SH) ranking basedon toxicity, flammability, and/or reactivity. Notwithstanding many conventionaland well characterized toxic industrial chemicals (TICs)/toxic industrial materials

1http://factsindia.wordpress.com/category/standex/

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(TIMs), there are many other emerging, persistent, dual-use, and unconventionalthreat vectors that must be identified and mitigated using advanced technologicalplatforms. Despite its many useful applications, research on genetically modifiedorganisms (GMO) coupled with recent advances in nucleic acid synthesis, lackof in-situ detection, reference materials and standards have caused public andscientific concerns. Synthetic biology (syn-bio), likewise, is modifying the DNAof an organism to alter its coded information, such as selecting desired mutations,inducing mutagenesis, altering genes or regulatory elements, cloning options,designing biological components and systems that otherwise do not naturally exist.Syn-bio provides the tools to “redesign” the genomes of existing microbes toincrease their efficacy or offer brand new functionalities. As an example, thesuccessful “redesign” of the bacterium Mycoplasma genitalium, which has thesmallest known bacterial genome, yet possesses all of the bio-chemical machineryneeded to metabolize, grow, and reproduce, has been readily available in openliterature and on the Internet [citation withheld]. Syn-bio is subject to potentialmisuse, in terms of its enhanced virulence, resistance to control agents, alteringhost defense, and increasing environmental stability and/or dispersal. Similar toGMO, limited detection methods exist for emerging syn-bio threats requiringcountermeasures using advanced technology innovations platforms. Dual-use is aterm often used in the political and diplomatic context to express a technology thatcan be used for both peaceful and military applications. Although used primarilyin context of nuclear proliferation, the use of GMO and (bio) weaponized syn-bioagents pose a major concern. Hence, THz systems require imaging that use complexalgorithms for processing of information and strategic assessments and modelingof mixed and complex hazardous environments to delineate signal from noise andbackground interactions.

14.1.1 Sampled vs. Remote Detection

The two most important metrics for the sensing and transduction functions aretime to detect material under investigation to enable appropriate response andthe resulting response function consistent with the species detected. Given thecomplex environment described above, the challenges at subsystem levels occurto evaluate overall effectiveness and efficacy of the sensor/detector systems. Inpoint/direct/sampled detection platforms, the analysis is accurate, real-time (inmost cases), and conform to sensor/detector metrics of specificity, selectivity, andsensitivity. However, due to the extreme nature of some chemical-biological agents,it is not always feasible to have either a direct or close contact with such environ-ments. In such cases, stand-off detection/imaging systems are required. Stand-Offdetection/imaging systems consist of a set of methodologies adopted to detectCBRNE agents and contamination to provide fast, reliable, and real-time detectionand differentiation of chemical, biological (e.g., bacteria, virus, pathogens), volatileorganic compounds (VOCs), TICs/TIMs, returned/unused pharmaceuticals, and

14 New THz Technologies and Applications in Applications in Support. . . 281

other contaminants at a distance. Generally, optical properties of nanomaterialsare considered for stand-off detection/sensing/imaging applications when combinedwith biotechnology and quantum mechanics [2]. Several other approaches includeuse of nanoparticles, more specifically, quantum dots for catalysis or carriers toenable transduction [3]. A few examples are surface-enhanced Raman spectroscopy(SERS) [4] and localized surface plasmon resonance (SPR) [5]. Yet anotherapplication is in metamaterials or negative refractive index materials for potentialapplications in satellite imaging elements [2]. Potential challenges arise in that thesource of illumination must have sufficient light intensity with minimum powerconsumption. Further, intensity based measurements are susceptible to intensity-based noise in signals and require appropriate signal extraction software. Yet anotherstand-off detection methodology is generation of high frequency electromagneticwaves using carbon nanotubes (CNTs) that identify signatures in reflected/scatteredbeam of potential chemical-biological agents [6]. Significant challenges still remainin the generation of high frequency using CNTs and also producing CNTs ofsame chirality [7]. Electromagnetic signals at THz frequencies have the advantagethat their photon energies are in the low meV range and therefore not capable tobreak up organic molecules of living cells. Therefore they have been consideredfor many applications such as airport surveillance. Such scanners can detect metalssuch as weapons hidden under clothing from a distance of several meters, withoutexposing the persons to harmful radiation. There are many other applications,such as identifying the validity of documents by identifying THz metallic thin-film structures covered by only optically opaque paints. THz spectroscopy is underextensive investigation to be used to successfully characterize many materials,including packaging, explosives and drugs.

14.1.2 Signal Intelligence (SIGINT)

SIGINT2 is remote intelligence information gathering from communications intel-ligence (COMINT), electronics intelligence (ELINT), and/or telemetry intelli-gence (TELINT). Intelligence is derived from instrumentation signals consistingof a category of devices either individually or in combination. The intercep-tion/receiving/deciphering of such transmissions can provide information on thetype and location of even low power transmitters. Most military communications arerestricted by encryption algorithms. To de-encrypt, complex processing is requiredin conjunction with additional layer of intelligence to analyze patterns and contentsof transmissions over time. Generally speaking, electronic intelligence (ELINT) isanalysis of non-communications electronic transmissions to include TELINT and/orradar transmitters (RADINT).

2http://factsindia.wordpress.com/category/standex/

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Generating an Electronic order of battle (EOB) (covers both COMINT andELINT) requires identifying SIGINT emitters in an area of interest, determiningtheir geographic location or range of mobility, characterizing their signals, and,where possible, determining their role in the broader organizational order of battle.The Defense Intelligence Agency (DIA) maintains an EOB by location and theJoint Spectrum Center (JSC) of the Defense Information Systems Agency (DISA)supplements this location database with five more technical databases: FrequencyResource Record System (FRRS); Background Environment Information (BEI);Spectrum Certification System (SCS); Equipment Characteristics/Space (EC/S);and Tactical Database (TACDB) – i.e. platform list sorted by nomenclature. Acomprehensive analysis is beyond the scope of this publication, however, it isemphasized that THz plays an incremental role in SIGINT and measurement andsignature intelligence (MASINT). THz, although in its initial stage, has the potentialto become a dominant feature of modern warfare. With shifting operational spec-trum, the term electronic warfare (EW) is likely to be changed to electromagneticwarfare (EMW) – to effectively suppress the use of communication channels ofadversaries, MASINT, optimizing its use by friendly forces, and remote plumeanalysis.

14.2 THz Imaging Technology – Basic Operation of Systemsand Phenomenology

The following sections provide an overview of the latest information of THztechnology, system concept implementation, and its implementation in terms ofsafety and security in the battlefield. THz region, as shown in Fig. 14.2, is afundamental frequency range having origin in rotational and vibrational modes ofmolecules. Transition between rotational modes with angular momentum l�1 and l,where the symbols have their usual meaning,

�El D El�1 �El D l .l C 1/ �2

2�r20� l .l � 1/ �2

2�r20D l�2

�r20

with,

m1r21 Cm2r

22 D �r20

provides, 4El � few meV, which overlaps with the intermolecular vibration modesof many known and potential explosives.

The electromagnetic spectrum from mm to THz can be used to create an imageof an object and to gather information on its chemical composition by measuringthe absorption (or refection) of electromagnetic energy simply by measuring theintensity of reflected or emitted energy. Generally, the imaging technique consists

14 New THz Technologies and Applications in Applications in Support. . . 283

Fig. 14.2 Upper panel: Electromagnetic spectrum showing THz range occurring at the inter-section of electronic and optical region. Tunnel injection transit-time (TUNNETT), super-latticeelectron device (SLED), IMPact ionization Avalanche Transit-Time (IMPATT), resonant tunnelingdiode (RTD), HG, quantum cascade (QC) laser. Lower panel: THz range showing regions of intra,inter molecular vibration, stretching, bending and absorption bands of several materials of interest

of two classes – active and passive. Active imaging systems illuminate the detectionspace with a beam of THz power, either by illumination of the entire space or as afocused beam scanned over the project, with detectors specifically sensitive to theilluminating frequencies. Since the energy passes through most materials but onlyto a skin depth, hence potential adverse health impacts are significantly less thanthose from the competitive imaging technologies using x-rays. In passive imaging

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Fig. 14.3 Imaging methodologies of THz – (a, b) time domain spectroscopy (TDS), (c) direct(passive) imaging, and (d) heterodyne imaging

detection techniques, the methods rely on collecting naturally occurring radiationand using the emissivity contrast between objects.

Figure 14.3 shows basis of several active and passive THz imaging methods.Time domains spectroscopy (TDS) as shown in Fig. 14.3a, b is used to focus THzbeam to an intermediate focus using a pair of lenses or parabolic reflectors, whichare inserted into the region where the THz beam is collimated.

An object of interest (to be scanned) is placed at the focus and then theamplitude and delay of the wave that has traversed through the object is measured.By translating the object and measuring the transmitted THz waveform for eachposition of the object, a pixel by pixel image is formed. TDS in reflection modeis shown in Fig. 14.3b. Usually, materials with high absorption coefficients arebetter suited for reflection geometry and those with low absorption coefficients fortransmission geometry.

The detector provides a current signal proportional to the electric field but not tothe intensity, which allows the determination of the absorption coefficient and therefractive index of the sample. Heterodyning, the beating together of two closelyspaced frequencies to yield the sum and difference of the original signals, has beenin use since the early days of radio. The primary advantage is the acquisition ofvery weak, narrow band signals where direct detection and post amplification (or

14 New THz Technologies and Applications in Applications in Support. . . 285

Table 14.1 Objectemissivity

Object Emissivity %

Human skin 65–95Plastics 30–70Paper 30–70Ceramics 30–70Water 50Metal Approx. 0

even pre-amplification) followed by detection, adds electronic noise to a point thatextracting signal needs long integration times. Heterodyne imaging, Fig. 14.2d,can be useful for both passive or active imaging, and in fact, in many instancescan simultaneously employ the same system for both observing modes by simplyturning off (or chopping) the coherent illumination source. Many different systemsare used based on commercially available components and hence it is not withinthe scope of this publication to simply review all of them. Nevertheless, there iswide range of applications under consideration; however the scope here is limitedprimarily to safety and security.

Most imaging techniques rely on the contrast of temperature or emissivity.Passive systems use natural background radiation for the illumination of detectionspace. Every object generates EM emissions at all wavelengths with intensityproportional to the product of its physical temperature and its emissivity inaccordance with Planck’s radiation law. Passive imaging systems require that therebe an apparent temperature difference, either positive or negative, between the bodyand its surroundings. While the surrounding environment is generally colder thanthe human body, some passive systems use non-coherent sources that surroundthe body to enhance contrast by making reflective objects appear warmer than thebody. The passive detection systems require the ability to differentiate a temperaturedifferential. Similar to a camera, a passive THz imager, as shown in Fig. 14.2cis able to image concealed weapons based on the implementation of detectionhardware. For comparison, the Table 14.1 above lists objects emissivity. Severalof such systems are in developmental stage and/or in the process of being deployed.

14.3 System Concepts and Implementation Strategies

For this report, the system requirements are limited in scope to the operationalneeds based on safety and security. The system configuration is further based ona narrow premise of explosive detection in a stand-off configuration in support ofconcealed-object identification, primarily for transport security; stand-off detectionof explosives; and measurement and signature intelligence – albeit very broad, thepremise includes picking up “a” signature from distance and analyzing the receivedinformation. Many different variations are envisaged and possible.

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Primarily, the THz system-development activity is to primarily develop a tech-nology to accomplish the following tasks:

(a) Stand-off detection of the presence, location, and identification of weapon,explosive compounds and devices, and other items of interest.

(b) Distant monitoring of a person carrying metallic objects, weapons, explosivematerials and devices, other items of security interest, while maintaining privacyof all individuals.

(c) Stand-off detection of plume of TICs/TIMs, biologics, and other agents ofinterest. Collection and analysis of signatures of various compounds andcapability to conduct real-time analysis. An extension of the project is to gatheran electronic signature to analyze, jam/defeat if necessary, and other inferencesthat can be derived from SIGINT, in support of MASINT and HUMINT.

The principal system components are provided below:

(a) A detector array and/or scanning device(b) Image acquisition – software/hardware(c) Image interpretation/recognition/computation(d) Database containing images/spectra for comparison(e) Display hardware/devices(f) Network interface and other elements of the layered system

The system development depends on the requirement, taking into accountseveral trade-offs between design related issues and performance requirements. Infact, there exist many guidelines for various commercial systems presently underdevelopment. With the advancement of new technologies, such as nanotechnologies,many new configurations are possible. The discussion below is limited to systemcapabilities and how new, advanced, and nanotechnologies can improve the systemperformance.

The process of identification begins with detection followed by recognition andclassification using image-recognition algorithm and matching it with items ofinterest. The recognition process goes through several hierarchical thresholds foridentification and matching with high probability of occurrence and low probabilityof false recognition. Such steps are based on reflective properties of substances, asshown below in Table 14.2, showing reflective properties of basic explosives, andhuman flesh.

This data is further normalized with reflection of clothing material, which isalso stored in the database. The final assessment of identification is difficult andrequires creative and innovative algorithm design. Many active millimeter waveimaging systems are commercially available, such as Pacific Northwest NationalLaboratory3, Safe-view by L3 communication4, QinetiQ5, Agilent technologies6,

3http://www.technet.pnnl.gov/sensors/chemical/projects/ES4THzSpec.stm4http://www.sds.l-3com.com/5http://www.qinetiq.com/Pages/default.aspx6http://thznetwork.net/index.php/archives/1813

14 New THz Technologies and Applications in Applications in Support. . . 287

Table 14.2 Reflective properties of basic explosives, and human flesh

Molecular Density Dielectric Reflectivity

Substance/name weight (g/cm3) constant Reflectance (R) Decibel (�dB)

2,4,6 Trinitrotoluene(TNT)

227:13 1.65 2:7 �0.24 12:3

Hexahydro-1,3,5-trinitro-1,3,5-triazine(RDX)

222:26 1.83 3:14 –0.28 11:1

Cyclotetramethylene-tetranitramine(HMX)

296:16 1.96 3:08 –0.27 11:2

Pentaerythritoltetranitrate (PETN)

316:2 1.78 2:72 –0.25 12:2

2,4,6-Trinitrophenyl-N-methylnitramine(Tetryl)

287:15 1.73 2:9 –0.26 11:7

Nitroglycerin (NG) 227:09 1.59 19 –0.63 4:1

Ammonium nitrate (AN) 80:05 1.59 7:1 –0.45 6:9

RDX TNT (COMP B) 2:9 –0.26 11:7

RDX (COMP C-4) 3:14 –0.28 11:1

PETN (Detasheet) 2:72 –0.25 12:2

HMX TNT (Octol) 2:9 –0.26 11:7

RDX-PETN (Semtex-H) 3 –0.27 11:4

Human flesh(H2OCNacl)

0.93 88 –0.81 1:9

Millitech7, Trex Enterprise Corporation8, Brijot9 and Millivision10, as some of theprimary vendors.

As the THz systems are currently under development, some of the latesttechnologies are considered with objective that such THz sources will ultimatelyprovide;

(a) Nondestructive inspection through dielectrics using TTDS pulse techniques,(b) Medical diagnostics through skin or thin tissue for non-intrusive purposes,(c) Detection of undesired metals and contraband hidden underneath clothing,(d) Emission of signal detected/identifies from a finite distance.

With the reduced dimensional materials, new and unique characteristics evolveand are considered for integrations from systems perspectives [2]. The first focusis on THz generation such that relatively large signal powers can be realized.One of the configurations is based on an electron resonance structure formed by

7http://www.millitech.com/8http://www.trexenterprises.com/9http://www.microsemi.com/products/screening-solutions (redirected)10http://www.microsemi.com/products/screening-solutions (redirected)

288 A. Vaseashta

a semiconductor hetero-junction structure [8]. The electrons in n-layer needs tobe accelerated by Ve – applied alternating voltage, such that they reach (lengthL� 150 nm) the barrier of the high-mobility n-type wide bandgap semiconductorand are reflected back without a loss of kinetic energy. The electrons then traveltowards the opposite barrier ballistically, where they are reflected again. When Ve

changes the polarity, this process continues resulting in electron resonance produc-ing THz signals. An extension of the ballistic device is the use of simultaneous D.C.biasing and the generation of mobile electrons in a short quantum well of smallergap values by a pulsed optical signal. The optical pulses can be longer than half ofthe THz period due to space charge limited effect of electron bunch transfer.

Another configuration under consideration is a heterostructure equivalent to thewell-known step recovery diode, arranged as a double structure. This provides a newnonlinear device configuration for highly efficient harmonic THz signal generation.In fact, two barriers opposing each other via narrow-gap semiconductors, such asAlGaAs/GaAs/AlGaAs or InAlAs/InGaAs/InAlAs with a proper n-doping can beconsidered as two step-recovery-diode junctions in opposition, which functionswithout any applied D.C. bias requirement. To generate THz radiation using aset of suitable materials as targets, the optical excitation can be provided [8] bytypically a 12-fs mode-locked Ti:Sapphire laser of center frequency �790 nm andrepetition rate �75 MHz. The emitted THz radiation is detected either using apneumatic Golay cell (incoherent detection) or in a conventional TDS arrangementusing electro-optic (coherent) detection.

New types of THz generators are based on field emission of electrons intovacuum using carbon nanotubes (CNTs) based structures that are capable ofproducing high current densities (based on assumptions of field amplification factorand estimation of electron work function) [9]. Industry requires a miniaturizedsource to enhance speed and resolution of the scanning process. A portable THzsource for this frequency range uses the Dynatron oscillator concept based ontravelling-wave structures [10]. This is composed of a triode tube with a grid voltagehigher than the anode voltage. This configuration accelerates secondary electronsfrom the anode to the grid, which makes the dynatron to act as a negative resistancedevice. A serial or parallel oscillator circuit is linked between the anode and aworking point potential source using a lower value than at the extraction grid.

14.4 Forward Thinking and Pathways for FutureImplementation in Safety and Security

Due to advantages described above, the field of THz generation, detection, andimaging is still in the R&D infancy phase. Limited or no commercial or opera-tionally deployed systems exist as of yet, for security applications.

Many approaches are pursued and can be divided in the following categories toachieve desired applications, as shown in Fig. 14.4.

14 New THz Technologies and Applications in Applications in Support. . . 289

Fig. 14.4 Applications of THz imaging including imaging, detection, MASINT, and plumemonitoring

(a) Sub millimeter wave electronic components and system for single frequencyoperation between 300 and 600 GHz.

(b) Broad-band imaging and spectroscopy(c) Component and system development for systems above 600 GHz.

Sub millimeter wave systems are designed for security applications with elec-tronic source at 600 GHz for targeting at approx. few 10s of meters. Receiversmainly concentrate on heterodyne techniques. Above 600 GHz, the work is focusedprimarily at component level. Microbolometers are used for broadband solution fordetector arrays. In addition to being low cost solution, the devices can be used forwide range of frequency and temperature range – based on the sensitivity required.

290 A. Vaseashta

Pulsed THz system, as described earlier demonstrates stand-off explosive detec-tion using reflection spectroscopy at a distance of 1 m. Liu et al. [11] have shown acollimated THz beam with detection distance up to 30 m in air, however with opticalsystem placed in close proximity to the target to gather reflection spectra accurately.

Reduced dimensional systems offer many additional options and are consideredas “molecular scanners” using broadband THz at approximately 10 nm scale.Furthermore, “optical matrix codes” covered with an optically non-transparentcoat can be deciphered by THz waves. Such systems have potential for inventorymanagement, if such patterns can be developed on flexible substrates. Similarly,such paints can also cover currency for protection from counterfeit currency,deciphering confidential digital information, and reading digital authenticationof products. Also, a matrix of THz-reflecting metal antennas can be employed,where the presence of such structures involving confidential digital information isdeciphered by identifying the far-field reflection pattern by an array of receivingantennas.

Imaging techniques is a critical issue towards the realization of a THz camera. Toobtain Continuous Wave (CW) operation the main challenge is power handling viasuitable heat sinking strategies [12]. The image detection is then based on an arrayof photoconductive antenna heterodyne receivers illuminated with two phase-lockedoptical wavelengths obtained from an Optical Frequency Comb Generation (OFCG)for CW operation. This architecture allows for an efficient, phase controlled, LocalOscillator (LO) distribution with low losses intrinsic to the use of optical fibers, aswell as amplitude and phase recovery due to the coherence of the LO distributed.Tunable operation is achieved using the different lines of the OFCG and broadbanddesign for the photoconductive antenna. A recent development of the OFC sourcein combination with radiofrequency and photonic electronic technique has resultedin synthesizing very high quality signals, while reducing overall size11.

THz radiation can pass through clothing and packaging, but they are stronglyabsorbed by metals and many other inorganic substances. THz sources use a numberof basic techniques, namely either harmonic extraction from the mm-waves or usingvarious methods from the optical signals. The possibility of deep-infrared lasers byquantum-cascading reaches the low THz frequencies of interest only by cooling toliquid nitrogen or below.

THz however presents some drawbacks thus limiting its use in every day medicaluse. These limitations cover a wide range from the low-performance of emittingsources to the low sensitivity or selectivity to pathological tissues. Nanotechnology-based techniques seems to be a crucial key tool in their efforts to improve theseimaging modalities, by using nanoparticles as contrast agents. As mentioned earlier,CNT is a suitable candidate for compact THz source. Similarly, many nanomaterialssuch as quantum dots (QDs) based systems contain free electrons, thus due to theirintrinsic discreet energy level, long carrier relaxation times and the ability to controlthese times, nanomaterials offer the way forward for QD-based THz optoelectronic

11http://portal.uc3m.es/portal/page/portal/actualidad_cientifica/noticias/terahertz_luz_wavelabs

14 New THz Technologies and Applications in Applications in Support. . . 291

devices [13]. Furthermore, due to existence of surface plasmons, non-linear opticalphenomena are enhanced due to the strong interactions and the high field strengthsdue to SP excitation and multiphoton photoelectric effect.

14.5 Conclusion

After successful launch of millimeter wave systems, higher frequencies in theTHz range are now beginning to become more widely used in this field. Higherfrequencies offer compact systems, especially using materials in the reduceddimensions. Many components and systems are currently under development upto sub-millimeter to low THz frequency range. The current, THz system arestill at the R&D laboratory scale, nevertheless the envisaged applications rangefrom remote/stand-off weapon depletion, imaging concealed metal objects, andanalysis plume from a distance. Many other applications exist such as counterfeitcurrency detection, inventory management, protecting classified information, anddigital authentication of products to list a few. Before systems can be furtherdeveloped, additional research is required, both in source and detector technology.With additional emphasis on stand-off detection systems, use of nanomaterial basedsystems offer plausible solutions as light weight THz generators and detectors.THz continues to demonstrate a strong promise as a technique for screeningdue to its potential for material specific detection. Finally, with lighter weightdevices, it will be possible to mount such devices on a UAV for MASINT, plumemonitoring, and also for digital signature recognition and localized monitoring forCommand, Control, Communications, Computers, Intelligence, Surveillance andReconnaissance (C4ISR).

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