This PDF file contains the front matter associated with SPIE Proceedings Volume 8119, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

A photonic transceiver module integrating a uni-traveling-carrier photodiode, a Schottky barrier diode, and a planar circulator circuit has been developed for sub-terahertz(THz)-wave reflection-geometry imaging. All the components are assembled in a compact rectangular-waveguide-output package for operation in the J-band (220 - 325 GHz), and continuous sub-THz waves are generated by the photomixing. The frequency dependence of the detection sensitivity of the transceiver module exhibits clear resonant behavior at around 270 GHz. The peak internal signal-to-background (S/B) ratio is measured to be as large as about 10. The characteristic of the fabricated transceiver is evaluated by measuring two-dimensional images of a test sample at frequencies from 240 to 310 GHz. Although the image resolution degrades with signal frequency deviation from the resonant condition due to the S/B ratio decrease, it is confirmed that a practical contrast can be obtained for a bandwidth of about 40 GHz even with an S/B ratio below one. Based on these results, the reflection-geometry in-vivo imaging of a human finger at 270 GHz is successfully demonstrated.

The aim of this paper was the verification of our identification model. The main assumption of our model was the possibility of the distinction between indicated compounds based on several narrow band detectors of terahertz radiation. Achieve of this frequency points was done with measurement data from FT-IR obtained in vacuum. The influence of absorption in water vapor was added numerically from Hitran simulation. The verification procedure was carried out with Time Domain Spectroscopy (for distance below 20 cm) and with narrow band sources (for distances above 20 cm). This verification was prepared in laboratory, controlled conditions. Our results confirm the possibility of application of the model in real THz stand-off security system.

The aim of this paper was to obtain the identification model in refection mode. Results Time Domain Spectroscopy were used to prepare our algorithm. This study has focused on developing several feature extraction methods with intuitive justifications in the problem space. A related problem to feature extraction is that of feature selection. For this reasons this extraction and selection methods of THz spectra are introduced. Then a complete THz classification framework including feature extraction scheme and Mahalanobis classifier was presented. Our results confirm the possibility of application of the model in real THz stand-off security system.

We report a pulsed method to measure reflection and scattering from several samples with different degrees of surface roughness and material properties at terahertz frequencies. Reflection from a flat gold mirror shows that the full width half maximum (FWHM) of the terahertz beam angular spread is <4° for frequency range 0.2 THz to 3 THz with signal-to-noise of 65 dB. Measurement of a paper index card, used as a low scattering sample, shows that the reflection/scattering properties are essentially similar to the system signature response except for multiple reflections between the front and back surfaces of the sample. Sixty-grit sandpaper shows multiple scattering events with almost no signal reflected from the flat backing paper surface. Corduroy cloth shows periodic reflections in the time domain, which correspond to diffraction lobes in the spectral domain.

THz medical imaging has been a topic of increased interest recently due largely to improvements in source and detector technology and the identification of suitable applications. One aspect of THz medical imaging research not often adequately addressed is pixel acquisition rate and phenomenology. The majority of active THz imaging systems use translation stages to raster scan a sample beneath a fixed THz beam. While these techniques have produced high resolution images of characterization targets and animal models they do not scale well to human imaging where clinicians are unwilling to place patients on large translation stages. This paper presents a scanned beam THz imaging system that can acquire a 1 cm² area with 1 mm² pixels and a per-pixel SNR of 40 dB in less than 5 seconds. The system translates a focused THz beam across a stationary target using a spinning polygonal mirror and HDPE objective lens. The illumination is centered at 525 GHz with ~ 125 GHz of response normalized bandwidth and the component layout is designed to optically co-locate the stationary source and detector ensuring normal incidence across a 50 mm × 50 mm field of view at standoff of 190 mm. Component characterization and images of a test target are presented. These results are some of the first ever reported for a short standoff, high resolution, scanned beam THz imaging system and represent an important step forward for practical integration of THz medical imaging where fast image acquisition times and stationary targets (patients) are requisite.

Temperature effects in terahertz (THz) step well quantum cascade (QC) structures are investigated. Step well QC structures with diagonal optical transitions that use fast intrawell electron-longitudinal optical (LO) phonon scattering for depopulation are considered. A density matrix method is used to model the electron transport coherence and is incorporated into the Monte Carlo simulations of these structures. A phenomenological dephasing time is also included. The influence of the lattice temperature on the population inversion is modeled and the effects due to gain spectral broadening are also considered. Optical gain greater than typical waveguide resonator thresholds are estimated out to T ~ 200 K.

We discuss the generation of THz radiation at room temperature by the exploitation of a nonlinear optical process taking place in a high quality factor AlGaAs microcavity. The approach is grounded on 1) a novel quasi-phase-matching scheme for parametric processes involving whispering-gallery modes circulating in nonlinear microcylinders; and 2) recent advances concerning quantum dots microcylindrical lasers. After a brief summary of the theory used to describe the nonlinear process, we present the results of our modeling in the case of a passive device pumped by two lasers at wavelengths close to 1.3 μm. Finally, we conclude with preliminary measurements performed with a tapered fiber.

In the past decade, tremendous development has been made in GaAs/AlGaAs based THz quantum cascade laser (QCLs), however, the maximum operating temperature is still limited below 200 K (without magnetic field). THz QCL based on difference frequency generation (DFG) represents a viable technology for room temperature operation. Recently, we have demonstrated room temperature THz emission (~ 4 THz) up to 8.5 μW with a power conversion efficiency of 10 μW/W2. A dual-period distributed feedback grating is used to filter the mid-infrared spectra in favor of an extremely narrow THz linewidth of 6.6 GHz.

We demonstrate that azimuthally polarized micro-ring THz quantum-cascade lasers can be coupled to hollow metallic waveguides with efficiencies ≥96% and perfectly matched with the TE₀₁ waveguide mode, giving attenuation losses < 3dB/m. The use of Ag coated-flexible pipe waveguides allows propagation of either vertically polarized or azimuthally polarized QCL beams with propagation losses as low as 2.1-4.4 dB/m and bending losses lower than 1.2 dB.

We propose an experimental demonstration of a THz modulator with a visible optical command. The device is a n-doped GaAs grating with subwavelength dimensions. The principle of this modulator is the control of the THz resonant absorption by surface waves supported by the grating. This absorption is modulated with low power visible light, leading to a modulation of the reflected THz beam. From experimental polarized THz reflectivity measurement of the grating, we show that a depletion layer at the surface of the doped GaAs has to be taken into account to correctly describe the observed resonant absorption. From experimental observation and modeling we are able to ascribe this absorption to the coupling of incident THz light with surface plasmon-phonon polariton mode propagating along each wall of the grating. Thus, each wall acts as a nano-antenna that resonantly absorbs light. The grating can be viewed as a metamaterial composed of individual resonators. The theoretical model indicates that the reflectivity dip linked to the surface wave is sensible to the electronic density in the walls of the grating. We performed an experiment to measure the THz reflectivity while illuminating the grating with visible photons having energy higher than the bandgap of GaAs. The created photoelectrons change the effective mode index, leading to a shift of the resonant absorption frequency. This demonstrates the modulation of THz radiation around 8.5 THz with a visible optical command at room temperature.

Development of focal plane arrays (FPA) for mm wavelength and THz radiation is presented in this paper. The FPA is based upon inexpensive neon indicator lamp Glow Discharge Detectors (GDDs) that serve as pixels in the FPA. It was shown in previous investigations that inexpensive neon indicator lamps GDDs are quite sensitive to mm wavelength and THz radiation. The diameter of GDD lamps are typically 3-6 mm and thus the FPA can be diffraction limited. Development of an FPA using such devices as detectors is advantageous since the costs of such a lamp is around 30-50 cents per lamp, and it is a room temperature detector sufficiently fast for video frame rates. Recently a new 8×8 GDD FPA VLSI board was designed, constructed, and experimentally tested. First THz images as well as DSP methods using this GDD FPA are demonstrated. Super resolution was achieved by moving the 8×8 pixel board appropriately in the image plane so that 32X32 pixel images are also obtained and shown here, with much improved image quality because of much reduced pixelization distortion.

One of the modern problems arising in the detection and identification of substances is an influence of water vapour on the THz signal reflected from sample. The vapour of water distorts the signal and its spectrum. Hence, we get the signal, which contains information about water. We propose one of the possible approaches to avoid this influence on the detection and identification of substance. This approach concludes in full elimination of spectral lines corresponding to spectral lines of water absorption from the spectrum of the signal. Then we reconstruct the signal with obtained spectrum and make the spectral dynamics analysis for the identification of substance. The efficiency of our approach is demonstrated by its application for the THz signal reflected from explosive. It is essential that we analyze the THz pulse with a few cycles. As consequence, this pulse has broad spectrum. To prove the validity of our approach we make theoretical analysis of dependence of spectral dynamics at chosen frequency on neighbor frequencies. We show that only the medium response on some frequencies influences on spectral dynamics of chosen frequency. Hence, to analyze the substance response on chosen frequencies the necessity of analyzation of full spectrum is absent. It is very important step for real application of method of the spectral dynamics analysis (SDA-method) for the detection and identification of explosive.

Our work aims to identify nano-scale metal films with enhanced absorption between 1 to 10 THz for use in thermal imagers operating in this spectral band. Absorption measurements of chromium and nickel films with different thicknesses (8 - 30 nm) revealed absorption as high as 40% (Cr) and 27% (Ni) between 3 and 9 THz. Further analysis showed that it is possible to optimize absorption by controlling conductivity of metal films by patterning them to reduce fill factor. This design flexibility allows tailoring of the absorbing layer to reduce residual stress of membranes used in microbolmeter and bi-material thermal sensors.

When plasmas are instantaneously created around an electromagnetic wave, frequency of the wave up-converted to the frequency, which depends on the plasma frequency. This phenomenon is called as the flash-ionization predicted by S. C. Wilks et al [1]. The theory requires not only the plasma creation in time much shorter than an oscillation period of the electromagnetic wave but also plasma length much longer than a wavelength of it. We have demonstrated the proof of principle experiment using the interaction between a terahertz wave and plasmas created by an ultra short laser pulse, which ensures the plasma creation time-scale much shorter than a period of electromagnetic source wave and plasma length longer than a wavelength of the wave. We observed frequency upconversion from 0.35 THz to 3.3 THz by the irradiance of the Ti:sapphire laser in ZnSe crystal.

Strong beams of coherent radiation are essential to induce nonlinear excitation phenomena in biology and material sciences. Optical-field-induced ionization by an ultrashort laser pulse produces ultrabroadband bursts of radiation with photon energies ranging from radio-wave at the microsecond timescale to x-ray at the attosecond timescale. As the laser pulse drives an ultrafast-discharge with high current it induces nonlinear spectral conversion in a few femtoseconds and generates terahertz electromagnetic waves. Broadband terahertz generation has been reported in air and rare gases. If the radiation frequency depends on the electron plasma density, it should vary with the laser pulse intensity, and the kind and density of the gas. However, the peak radiation frequencies reported are almost independent of those parameters. From the laser-gas interaction point of view, the terahertz generation mechanism is not enough understood. We demonstrate a frequency-tuning scheme that uses the laser pulse duration to control the ultrafast-discharge current timescale, yielding a terahertz energy of 0.1 μJ and a conversion efficiency of 10^-4 by use of the homemade power supply with 60-A discharge current at 1 kHz. We also propose a simple physical model to explain the generation of terahertz radiation with the laser propagation in an ultrafast-discharge.