This PDF contains front matter associated with SPIE Proceedings Volume 8023, including Title Page, Copyright Information, Table of Contents, and the Conference Committee listing.

The terahertz (THz) spectral range offers promising applications in science, industry, and military. THz penetration through nonconductors (fabrics, wood, plastic) enables a more efficient way of performing security checks (for example at airports), as illegal drugs and explosives could be detected. Being a non-ionizing radiation, THz radiation is environment-friendly enabling a safer analysis environment than conventional X-ray based techniques. However, the lack of a compact room temperature THz laser source greatly hinders mass deployment of THz systems in security check points and medical centers. In the past decade, tremendous development has been made in GaAs/AlGaAs based THz Quantum Cascade Laser (QCLs), with maximum operating temperatures close to 200 K (without magnetic field). However, higher temperature operation is severely limited by a small LO-phonon energy (~ 36 meV) in this material system. With a much larger LO-phonon energy of ~ 90 meV, III-Nitrides are promising candidates for room temperature THz lasers. However, realizing high quality material for GaN-based intersubband devices presents a significant challenge. Advances with this approach will be presented. Alternatively, recent demonstration of InP based mid-infrared QCLs with extremely high peak power of 120 W at room temperature opens up the possibility of producing high power THz emission with difference frequency generation through two mid-infrared wavelengths.

The gapless and linear energy spectra of electrons and holes in graphene lead to nontrivial features such as negative dynamic conductivity in the terahertz spectral range. This paper reviews the recent advances in theoretical and experimental study on terahertz light amplification by stimulated emission of radiation in optically pumped graphene.

A density matrix and tight binding model along with a Monte Carlo approach are used to model electron transport in one and two-well terahertz (THz) step well quantum cascade (QC) structures. Two new structures were analyzed, a multi-step one-well structure and a principally two-well structure. Both of these structures use a diagonal optical transition for improved upper to lower lasing state lifetime ratio and feature a step well injector to provide near unity injection efficiency due to the spatial separation of the wavefunctions. Fast intrawell electron-longitudinal optical (LO)-phonon scattering is used to depopulate the lower lasing state which does not require the use of resonant tunneling. Density matrix Monte Carlo simulations are used to analyze these structures in order to investigate these properties. In these simulations scattering mechanisms including LO-phonon, electron-electron, impurity, and interface roughness scattering are treated semiclassically, while also contributing to dephasing scattering. A phenomenological dephasing time is also included to investigate the influence of dephasing on the electron transport within these structures. Subband populations, electron temperatures, optical gain, and current density are extracted from the simulations. The analysis indicates that it is necessary to include incoherent transport dephasing in order to provide realistic estimates of the transport process because the transport is primarily dominated by transitions between weakly coupled states. In addition, this analysis shows these simplified step well structures are capable of yielding high optical gain ~ 80 cm^-1 while at the same time expected to have relatively low threshold current densities |e|j ~ 380 A/cm².

The operating frequency range of millimeter wave Backward Wave Oscillators (BWOs) are extended to 2.2 THz using solid state frequency multipliers, enabling continuously tunable, narrow linewidth THz sources across the 0.1-2.2 THz range. Power conversion efficiency of frequency multipliers was improved substantially by optimizing impedance matching between the BWOs, multipliers, and free space. Impedance matching extracts more THz power out of BWOs coupled to frequency multipliers, than is emitted from BWOs radiating into free space.

In this work we review the most important results concerning the physics and applications of FETs as Terahertz detectors. We present two experiments showing: i) Terahertz detection based on low cost 130 nm silicon technology Field Effect Transistors in the sub-THz range (0.2 THz up to 1.1 THz) and ii) first results on detection by FETs of emission from 3.1 THz Quantum Cascade Lasers.

The structural, material and field dependence of the THz lasing frequency is reported for a QCL based upon GaN/AlGaN heterostructures. The inter-subband transition initiated generation of THz followed by the LO-phonon assisted fast depopulation takes into account the appropriate energy band alignments. Valence and conduction band alignments incorporating spin-orbit and crystal field splitting as well as bi-axial strain are used to determine the conduction band offset as a function of Al-mole fraction. Determination of eigen states takes into account the spontaneous and piezoelectric polarization induced modification in the conduction band profile. A lower THz generation frequency is predicted for Ga-face GaN/AlGaN-based QCLs using the revised energy band alignments.

Terahertz imaging has attracted much attention in recent years, because the technique can be applied to many application fields such as nondestructive analysis and imaging method through opaque materials. A terahertz real-time imaging technique (Terahertz Camera) considered increasingly important in the future has been developed. The terahertz camera consists of a light source (Terahertz quantum cascade laser) and an un-cooled micro-bolometer array, which can easily get real-time terahertz-image. As an application of the terahertz camera, a stand-off imaging system that could be useful in a fire disaster relief and a label-free bio-materials detection system have developed and demonstrated.

A THz camera based on an uncooled microbolometer 160X120 pixel array with nominal pitch of 52 μm has been developed at INO and initial transmission and reflection images showed promise. In the present paper, the characterization of both standard infrared and THz-optimized uncooled microbolometer pixel arrays are presented at both infrared and THz wavelengths. Measurements in the THz region has been performed using non-uniform low-power quantum-cascade laser (QCL) and uniform high-power far-infrared laser (FIR laser) beams at 3 THz and 4.25 and 2.54 THz, respectively. A measurement comparison has been achieved in the infrared using a blackbody radiation. Different methods for noise-equivalent power (NEP) measurements have been investigated. These characterization methods are promising especially for non-uniform laser beams irradiated on pixel arrays. The NEP results obtained from the different methods are in good agreement independent of the method used in the experiments. The results show a high sensitivity of the THz-optimized pixel array in the THz region. Large beam area reflection imaging of obscured materials at 2.54 THz have been performed at video rates of 30 frames per second using the THz-optimized pixel array equipped with a semi-custom fast THz objective, proving that the INO THz camera provides a promising solution for stand-alone imaging systems.

Passive millimeter wave imaging is very useful for security applications since it candetect objects concealed under clothing. In this paper,the multi-level segmentation of passive millimeter wave images is presented to detectconcealed objects under clothing. Our passive millimeter wave imaging system is equipped with a Cassegrain dish antenna and a receiver channel operating around 3 mm wavelength. The expectation-maximization algorithm is adopted to cluster pixelson the basis ofa Gaussian mixture model. The multi-level segmentation is investigated with different numbers of clusters in Gaussian mixture distribution. The performance is evaluated by average probability error. Experimentsconfirm that the presented method is able to detect the wood grip as well as metal part of the hand axconcealed under clothing.

Spectral signatures of solid materials in the THz range can provide spectroscopic information for chemical identification. Previously we have demonstrated the absorption coefficient extraction by scanned imaging of QCL THz beams attenuated through explosive samples. The detection was achieved by a unique pixel addressed within an uncooled antenna-coupled microbolometer 160x120 array specifically designed for the 1-5 THz range. This detector technology developed at CEA-LETI relies on amorphous silicon bolometer know-how and aims at opening the way to real-time video rate, with potential low cost. We report complementary tests of imaging in reflection configuration and the first tests of a second prototype where 320x240 bolometers are monolithically processed above a CMOS read-out circuit.

The widespread adoption of THz based applications has been hindered by the lack of a real-time, broad-band, cost-effective THz camera with sufficient sensitivity to enable applications in markets as diverse as security, non-destructive evaluation, and biomedical imaging. This technological gap can be filled through the development of an 80 x 64 pixel array of Sb-heterostructure backward diodes (Sb-HBDs) monolithically integrated to broadband (600 GHz - 1200 GHz) antennas that can be directly flip-chipped to a CMOS voltage-mode readout integrated circuit (ROIC). This paper outlines the current progress of the project.

Broadband sub-millimeter wave technology has received significant attention for potential applications in security, medical, and military imaging. Despite theoretical advantages of reduced size, weight, and power compared to current millimeter-wave systems, sub-millimeter-wave systems are hampered by a fundamental lack of amplification with sufficient gain and noise figure properties. We report on the development of a sub-millimeter wave amplifier module as part of a broadband pixel operating from 300-350 GHz, biased off of a single 2V power supply. Over this frequency range, > 38 dB gain and < 8.3 dB noise figure are obtained and represent the current state-of-art performance capabilities. The prototype pixel chain consists of two WR3 waveguide amplifier blocks, and a horn antenna and diode detector. The low noise amplifier Sub-Millimeter-wave Monolithic Integrated Circuit (SMMIC) was originally developed under the DARPA SWIFT and THz Electronics programs and is based on sub 50 nm Indium Arsenide Composite Channel (IACC) transistor technology with a projected maximum oscillation frequency f_max > 1.0 THz. This development and demonstration may bring to life future sub-millimeter-wave and THz applications such as solutions to brown-out problems, ultra-high bandwidth satellite communication cross-links, and future planetary exploration missions.

Sensitive water concentration mapping in thin animal tissue samples has been demonstrated using tunable monochromatic THz-wave parametric source. A novel sample preparation approach is performed to effectively preserve tissue freshness at room temperature. The time course results show the sample characteristic of water content and distribution can be well measured and excellently repeated in 70minutes with a standard deviation of less than 1%. These results suggest the method of water volume concentration and distribution measurement using THz-wave has good stability with proper sample preparation, which has great potential in the fields of medical and biological diagnosis.

We have developed ultra-widely tunable THz-wave source using organic nonlinear optical crystals such as 4- dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) and N-benzyl-2-methyl-4-nitroaniline (BNA). The THz-wave difference frequency generation using these crystals covers the ultra-widely tunable range of 0.1-40 THz with frequency agility. Collaborating with Furukawa Co. Ltd., we used the progressive, frequency-agile THz-wave source for industrial applications and produced a sensitive, non-destructive method for examining carrier-density and electrical properties of semiconductors. This method presents novel possibilities for use in the semiconductor industry.

Nearly every wavelength from dc to x-rays significantly benefits today's society with a notable exception: the THz or sub-millimeter band. Applications in this range are widespread, penetrating fields ranging from national security to medicine, but remain largely under-utilized due to the difficulty of creating, manipulating, and detecting this type of radiation. Our research focus is THz source development using superconductive Josephson junctions to potentially address the lack of compact, efficient, and tunable source of continuous THz radiation. We review relevant applications for such a source and report an optimized design of a miniature THz emitter using single crystal intrinsic Josephson junctions.

Resonant cavity enhancement results in substantial improvement in the efficiency of photonic THz-wave generation via frequency down conversion. Efficient THz wave generation was demonstrated at 2.8 THz previously by difference frequency mixing between resonating signal and idler waves of the linear-cavity type-II-phase-matched PPLN optical parametric oscillator (OPO). A new, simplified approach to resonantly-enhanced THz-wave generation in periodic GaAs, featuring (i) ring, instead of linear, OPO cavity with much higher finesse, (ii) type-0, instead of type-II-phase-matched PPLN crystal as a gain medium, resulting in much lower OPO threshold, (iii) a compact picosecond 1064-nm fiber laser as a pump source, and (iv) the use of a thin intracavity etalon with a free spectral range equal to the desired THz output frequency is presented here. Intra-cavity THz generation was performed by 2.1 μm anti-reflection coated stacks of optically contacted GaAs wafers (OC-GaAs) and diffusion bonded GaAs wafers (DB-GaAs) with periodic-inversion placed in the second OPO focal plane. Using 6.6 W of average pump power, narrowband output in the range 1.4 - 3 THz was produced with more than 130 microwatts of average power at 1.5 THz. By optimizing the OPO PPLN crystal length and spectral characteristics of the fiber pump laser and OPO the demonstrated approach can be extended to generate 1-10 mW of THz output in a compact setup.

We examine InP and InGaAs Schottky type photoconductive antenna for the THz generation excited by femtosecond laser(fs) at a wavelength of 1560nm. Since InP has an energy gap of 1.3 eV, which is much larger than photon energy of fs laser. We obtained THz wave generation from both PC antennas with sufficient THz amplitude, which is comparative to that of low temperature grown GaAs.We also developed new type of InGaAs PC antennas which includes insulating gap in a PC structure.Highly bright THz beam was generated from the InGaAs PC antennas.

Recently, terahertz spectroscopy has attracted increasing interest as a probe of organic molecular crystals such as pharmaceutical compounds and explosives. Terahertz time-domain spectroscopy (THz-TDS) focuses on low-frequency (10-120 cm^-1) intermolecular modes that are characteristic of the crystal structure. The prospect of material-specific identification, combined with the ability of THz light to penetrate many common materials, has lead to an emphasis on the prospect of a THz-based, standoff explosives detection device. In this presentation, THz spectra of materials ranging from military-grade explosives to home-made explosive components will be discussed. In addition to the experimental results, THz spectral assignments will be presented based on solid-state density functional theory simulations. Finally, the effect of particle size on the observed spectra will also be considered.

The subject of this article is implementation of terahertz remote sensing for detection and imaging of concealed objects from distances of several metres. Many materials used for packaging and clothing are partially transparent in the spectral range 0.1 - 10 THz. The transparency property can be utilised to detect objects concealed by the materials, which are often opaque in other spectral regions. This can be achieved by detecting the radiation from these objects through the use of an appropriate detector, which is sensitive at THz frequencies. The radiation from the concealed objects can be either self-emitted or reflected. The use of THz remote sensing is being pursued in IARD by both theoretical and practical approaches. The article contains a short review on the detectors, sources and components, which can be used for remote sensing systems operating at THz frequencies, and describes energy calculations and system design considerations. Characteristic and exemplar performance of the components, which are being used in IARD, is presented. The article then describes prototypes of a passive THz radiometer and an active THz system, which were built in IARD. Performance characteristics of both systems are described. The measurement results of the optical properties of various materials are presented as well as examples of images obtained by the active THz system.

The method of spectral dynamics analysis (SDA-method) is used for the detection and identification of explosive RDX and harmless substances in pellets with different weight and hidden under the thin layer of polyethylene or cotton by reflected THz signal. The signal was measured on the long-term interval with duration about 180 ps and contains not only the main reflected THz pulse but the subpulse with less intensity. Two cases of the reflection were investigated - at 90° and at 45° from the sample. SDA method allows to obtain the spectrogram - 2D THz signature of the substance, and to analyze the dynamics of many spectral lines of the signal by one set of measurements. The identifiers characterizing the presence of substance in the sample were found in all investigated samples - they contain in the corresponding spectrograms and dynamics of spectral lines. We showed that the analysis of the reflected signal over the short time intervals (less than 50 ps) is insufficient for reliable identification of the substances, especially of hidden ones, because in some cases the characteristic features of the substances can be found only in the subpulses. The influence of window shape in Fourier-Gabor method on the dynamics of spectral lines was discussed too.

In this work, the performance of InP-based HEMTs as a THz detector was experimentally studied. The nature of the THz rectification by the two-dimensional plasmons in which the DC drain current variation ΔI_d becomes maximal around the threshold voltage was observed. Based on the imaging measurement, it was confirmed that our HEMTs device can work for sensitive THz imaging at 0.3 THz. The directivity of the detector was characterized with the maximum responsivity of 26.1 V/W at θ = 160 degrees.

Recent progress of laser terahertz (THz) emission microscope (LTEM) is reviewed.Femtosecond lasers can excite the THz waves in various electronic materials due to ultrafast current modulation. The current modulation is realized by acceleration or deceleration of photo-excited carriers, and thus LTEM visualizes dynamic photo-response of substances. The developed systems have a minimum spatial resolution better than 1.0 μm, which is defined by the laser beam diameter. We alos provide esamples of LTEM application to material science and semiconductor devise failure.

Tunable resonant absorption by plasmons in the two-dimensional electron gas (2DEG) of grating-gated HEMTs is known for a variety of semiconductor systems, giving promise of chip-scale frequency- agile THz imaging spectrometers. In this work, we present our approach to measurement of electrical response to millimeter waves from backward-wave oscillators (BWO) in the range 40-110 GHz for InP-based HEMTs. Frequency-modulation of the BWO with lock-in amplification of the source-drain current gives an output proportional to the change in absorption with frequency without contribution from non-resonant response. This is a first step in optimizing such devices for man-portable or space-based spectral-sensing applications.

Characterization of materials with THz waves is not trivial at the moment. Expensive and often bulky equipment, usually laboratory (rather than portable) set-up realization, possible water content, low depth of penetration, etc. are some typical problems. As a result, the desired characteristics of the material are not reliable and often difficult to obtain. In this situation, computational methods may be helpful in alleviating the above difficulties. In the long run, sophisticated mathematical model building may make THz characterization devices more suitable for field applications, implementing portable THz devices and set-ups as well as minimizing the measurement error without repetitive measurements. The computational methods based on time series analysis are described and results provided.

We present calculations of ground state resonance structure associated with the high explosive PETN using density functional theory (DFT), which is for the construction of parameterized dielectric response functions for excitation by electromagnetic waves at compatible frequencies. These dielectric functions provide for different of types of analyses concerning the dielectric response of explosives. In particular, these dielectric response functions provide quantitative initial estimates of spectral response features for subsequent adjustment with respect to additional information such as laboratory measurements and other types of theory based calculations. With respect to qualitative analysis, these spectra provide for the molecular level interpretation of response structure. The DFT software NRLMOL was used for the calculations of ground state resonance structure presented here.

The observation of THz regime transmission resonances in an InGaAs/InP high electron mobility transistor (HEMT) can be attributed to excitation of plasmons in its two-dimensional electron gas (2DEG). Properties of grating-based, gate-voltage tunable resonances are shown to be adequately modeled using commercial finite element method (FEM) software when the HEMT layer structure, gate geometry and sheet charge concentration are taken into account. The FEM results are shown to produce results consistent with standard analytical theories in the 10-100 cm^-1 wavenumber range. An original analytic formula presented here describes how the plasmonic resonance may change in the presence of a virtual gate, or region of relatively high free charge carriers that lies in the HEMT between the physical grating gate and the 2DEG. The virtual gate and corresponding analytic formulation are able to account for the red-shifting experimentally observed in plasmonic resonances. The calculation methods demonstrated here have the potential to greatly aid in the design of future detection devices that require specifically tuned plasmonic modes in the 2DEG of a HEMT, as well as giving new insights to aid in the development of more complete analytic theories.

In this paper, we propose a plasmonic parametric oscillator. The device is based on coupling between optically generated and electrically induced surface plasmon waves. The device can be used for a variety of applications involving spectrum analysis, widely tunable electromagnetic emitters, heterodyne detection, and amplification. We developed Maxwell's equations based on the theoretical model of the coupling between plasmons and electrically induced plasmons. Electrically induced plasmons are generated when a current is injected in the vicinity of a metallic grating. The coupling between the two kinds of plasmon bands is dependent on the skin depth of each. The skin depth of an optically generated plasmon is well known, while the skin depth of an electrically induced plasmon vanishes as the grating frequency becomes small or the injected current becomes large.

Quasi-optical and antenna-like ways of coupling free-space THz radiation into parallel-plate waveguide are examined. Cylindrical lenses made of cheap, off-the-shelf HDPE rods with 6, 10 and 16 mm diameter are used. A tapered input is also examined and it is found that in absolute terms the 16mm lens couples most radiation, but once the differences in effective aperture are accounted for, the tapered input is the best. 16mm and 6mm lenses proved to be sensitive to alignment and not very practical. FDTD simulations of the complete experimental setup are performed and show good qualitative agreement, especially inside the single-mode propagation region.