This PDF file contains the front matter associated with SPIE Proceedings Volume 7311, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Using the high penetrability of the terahertz waves and the characteristic absorption spectra in this frequency range, we built a non-invasive mail inspection system targeting drugs and explosives. The system is composed of two stages; in the first stage, the scattering of a continuous terahertz wave is used for selecting mail that contains concealed powder; in the second stage, the absorption spectrum of the suspicious mail is measured and the material is identified using a terahertz spectrum database. We evaluated the performance and the limits of the inspection system.

The terahertz (THz) portion of the electromagnetic spectrum provides specific spectroscopic information for substance identification. It has been shown that the spectral features of explosive materials might be used for detection and identification at stand-off distances. We report on the development of a THz spectrometer for explosive detection and identification. The system is based on THz quantum cascade lasers working at different frequencies. These are used for illumination of the object under test. The reflected and backscattered radiation from the object under test is detected with a sensitive heterodyne receiver. As a first step a single frequency, liquid-cryogen free heterodyne receiver operating at 2.5 THz has been developed. In order to realize maximum sensitivity a phonon-cooled NbN hot electron bolometric mixer with a quantum cascade laser as local oscillator were chosen. The concept of the system and first results will be presented.

Terahertz (THz) spectroscopy is a promising technique for the stand-off identification of hidden objects. The THz band is particularly well suited firstly because THz radiation penetrates many dielectrics like clothing and secondly because many potentially hazardous substances have characteristic signatures in the THz spectral region. In order to demonstrate the full potential of THz radiation for identifying possible hazards using characteristic signatures, different disturbing influences must be taken into account. We have performed experiments and simulations in order to investigate the possibilities and the challenges of terahertz stand-off identification. A special emphasis is paid on humidity in ambient air and properties of the sample like surface roughness and orientation with respect to the incident THz beam. Water vapor absorption strongly affects the THz spectra. Since the absorption lines are strong and narrow, the calculation must be precise. We have checked models well-known in meteorology covering the infrared and the microwave regions of the electromagnetic spectrum and achieved an accurate description of the measured THz spectral absorption using the program LINEFIT. The surface roughness of the sample strongly affects the bandwidth of the reflected spectra. Specular and diffuse reflection measurements using samples with different roughnesses have been used for determining the influences of different properties on the reflection spectra.

We describe the generation of terahertz optical frequency comb using the spontaneous emissions from a semiconductor optical amplifier as the signal source. The source drives an all-fiber LYOT-Sagnac birefringent fiber. This transforms the broad band source into a discrete set of evenly spaced frequencies. The output of the Sagnac loop mirror is then coupled to fiber pigtailed Faraday Mirror reflecting the optical frequency comb back into the optical amplifier for further amplification. The method resulted in generating optical frequency comb ranging from 183THz to 213 THz with frequency spacing of 560 GHz.

Recently, there has been a significant interest in Terahertz (THz) technology, primarily for its potential applications in detection of concealed objects as well as in medical imaging for non-invasive diagnostics. This region of the spectrum has not been fully utilized due to lack of compact and efficient THz sources and detectors. However, there are several reports recently on real-time THz imaging using uncooled microbolometer camera and quantum cascade laser (QCL) operating as a THz illuminator. The cameras used in these studies are optimized for infrared wavelengths and do not provide optimal sensitivity in the THz spectral range. The fabrication of microbolometer focal plane arrays (FPAs) is relatively complex due to the required monolithic integration of readout electronics with the MEMS pixels. The recent developments in bi-material based infrared FPAs, utilizing optical readout, substantially simplifies the FPA fabrication process by decoupling readout and sensing. In this paper, design and fabrication of a bi-material based FPAs, optimized for the THz wavelengths, as well as design and integration of the readout optical system for real-time imaging will be described.

Laser induced Semiconductor Switches (LSS), comprised of a gap antenna deposited on a semiconductor substrate and photoexcited by a pulsed laser, are the primary source of THz radiation utilized in time-domain spectroscopy (TDS). THz-TDS applications such as standoff detection and imaging would greatly benefit from greater amounts of power coupled into free space radiation from these sources. The most common LSS device is based on low temperature-grown (LT) GaAs photoexcited by Ti:sapphire lasers, but its power performance is fundamentally limited by low breakdown voltage. By contrast, wide band-gap semiconductor-based LSS devices have much higher breakdown voltage and could provide higher radiant power efficiency but must be photoexcited blue or ultraviolet pulsed lasers. Here we report an experimental and theoretical study of 10 wide band-gap semiconductor LSS host materials: traditional semiconductors GaN, SiC, and ZnO, both pristine and with various dopants and alloys, including ternary and quaternary materials MgZnO and InGaZnO. The objective of this study was to identify the wide bandgap hosts with the greatest promise for LSS devices and compare their performance with LT-GaAs. From this effort three materials, Fe:GaN, MgZnO and Te:ZnO, were identified as having great potential as LSS devices because of their band-gap coincidence with frequency multiplied Ti:Sapphire lasers, increased thermal conductivity and higher breakdown voltage compared to LT-GaAs, as well as picoseconds scale recombination times.

This paper studies the role of plasmonic modes for guided-wave propagation of THz/far infrared in metalclad planar waveguides, including metal-dielectric interfaces, dielectric-loaded metal slabs and parallel plate waveguides. The dispersion of modal characteristics of the plasmonic guided waves, such as the effective index, attenuation constant and the field confinement, as a function of geometrical features for different consisting materials and wavelengths are examined. Moreover, comparison is made between the THz plasmonic modes to their optical counterparts at visible/near infrared within the similar physical structures. Peculiar features of each structure are highlighted and regimes of interest are distinguished.

Pronounced resonant absorption and frequency dispersion associated with an excitation of collective 2D plasmons have been observed in terahertz (0.5-4THz) transmission spectra of grating-gate 2D electron gas AlGaN/GaN HEMT (high electron mobility transistor) structures at cryogenic temperatures. The resonance frequencies correspond to plasmons with wavevectors equal to the reciprocal-lattice vectors of the metal grating, which serves both as a gate electrode for the HEMT and a coupler between plasmons and incident terahertz radiation. The resonances are tunable by changing the applied gate voltage, which controls 2D electron gas concentration in the channel. The effect can be used for resonant detection of terahertz radiation and for "on-chip" terahertz spectroscopy.

The large aperture Fresnel zone plate antenna has been extensively investigated in recent years for application at frequencies from the microwave range to the terahertz region. These zone plate antennas have a focal length (F) and diameter (D) that are comparable (F/D = 0.3 to 2.5). The results of these investigations show that the phase-correcting zone plate gives performance comparable to a true lens. There is one limitation, however, in that most cases a secondary focus occurs on-axis at one-third the focal distance from the zone plate. The intensity is 10 dB or more down from the main focus. A method has now been developed to essentially eliminate this secondary focus by properly adjusting the amount and location of the phase-correcting grooves or rings. A revised positioning of the grooves and degree of correction can reduce the secondary focus, but maintain the integrity of the main focus. Specific examples have been analyzed and will be presented at the conference.

Recent advances in rapid prototyping technologies have resulted in build-resolutions that are now on the scales required for direct fabrication of photonic structures in the gigahertz (GHz) and terahertz (THz) regimes. To demonstrate this capability, we have fabricated several structures with 3D bandgaps in these spectral regions. Characterization of the transmission properties of these structures confirms the build accuracy of this fabrication method. The result is a rapid and inexpensive fabrication technique that can be utilized to create a variety of interesting photonic structures in the GHz and THz. We present the results of our characterization experiments and discuss our current efforts in extending the technique to fabrication of other structure types.

The performance of a high temperature superconducting junction detector is evaluated. The detector has been built to explore applications of terahertz imaging. The detector device is a high-temperature superconductor (HTS) Josephson junction, which is integrated with a thin-film ring-slot antenna. The ring-slot antenna is patterned on a magnesium oxide (MgO) substrate which is compatible with the detector's YBCO superconducting material lattice. A hyper-hemispherical lens made from high resistivity float zone silicon (HRFZ-Si) is mounted on the rear side of the substrate. The lens couples energy from an imaging system onto the antenna which couples the energy into the device. An existing terahertz imaging system is used in conjunction with the detector to allow for the exploration of relevant applications. The imaging system is based on a conventional quasi-optical design with a backward-wave oscillator as the source and raster scans samples for image acquisition. The imaging capability of the system has been assessed by trialing a range of applications in both transmission and reflection modes. Applications explored include imaging concealed weapons in packaging, non-destructive testing of materials, and imaging devices through plastic structures. The results generated by the imaging system demonstrate its capability for these applications.

Voltage-tunable plasmon resonances in a InGaAs/InP high electron mobility transistor (HEMT) are reported. The gate contact consisted of a 0.5 micron period metal grating formed by electron-beam lithography. Narrow-band resonant absorption of THz radiation was observed in transmission in the range 10 - 50 cm^-1. The resonance frequency red-shifts with increasing negative gate bias as expected. Photo-response to a tunable far-IR laser is reported. The device may have application in high-frame-rate THz array detectors for spectral imaging with real-time chemical analysis.

This paper presents an algorithm for detecting handguns in terahertz images. Terahertz radiation is capable of penetrating certain materials which are opaque at optical wavelengths, such as clothing, without the harmful effects of ionizing radiation. The approach taken is to segment objects of interest and classify them based on shape. We use a modified version of an active contour algorithm found in the open literature. Modifications include: a pre-processing step that includes clutter filtering and seeding of an initial contour; and a post-processing step that removes clutter pixels from the segmentation. The features used for classification are moment-based and Fourier shape descriptors. Classification as handgun or non-handgun from these features is done via Fisher's linear discriminant.

We report on the transmission and reflection terahertz (THz) spectra of the high explosives RDX and PETN. These common military explosives are compared to simulants L-tartaric acid and sucrose, respectively. The use of these simulants enables researchers to develop many aspects of THz spectroscopy for explosives detection without the need for live explosives. Further, we discuss the effect of sample preparation on the THz spectrum of RDX and demonstrate that experiments performed on different terahertz instruments at multiple laboratories show quantitative agreement between spectra recorded with four different instruments.

A prototype THz imaging system based on modified uncooled microbolometer detector arrays, INO MIMICII camera electronics, and a custom f/1 THz optics has been assembled. A variety of new detector layouts and architectures have been designed; the detector THz absorption was optimized via several methods including integration of thin film metallic absorbers, thick film gold black absorbers, and antenna structures. The custom f/1 THz optics is based on high resistivity floatzone silicon with parylene anti-reflection coating matched to the wavelength region of interest. The integrated detector, camera electronics, and optics are combined with a 3 THz quantum cascade laser for initial testing and evaluation. Future work will include the integration of fully optimized detectors and packaging and the evaluation of the achievable NEP with an eye to future applications such as industrial inspection and stand-off detection.

The Submillimeter-Wave Technology Laboratory (STL) at the University of Massachusetts Lowell has investigated the electromagnetic scattering behavior of various broadband absorbers. Several absorbing materials were tested in a compact radar range operating at a center frequency of 160 GHz. The polarimetric radar cross section was measured at elevation angles from 15° to 75°. In addition to the backscattering behavior, the normal incidence transmittance of the materials was evaluated.

We present a scanning THz-camera with active illumination. Three different fully electronic transceiving techniques are evaluated: The first employs a commercial 230-320GHz frequency-modulated continuous-wave system with a harmonic mixing detector. Its bandwidth allows a ranging resolution in the mm-range. The second one is based on heterodyne detection operating at 645GHz with sub-harmonic mixer and provides a dynamic range beyond 100 dB. Like in the first system, we employ local illumination (THz beam focused on observed pixel). The third one equals the second one, but utilizes global illumination of the scene. In all cases, the scanning optics consists of a Cassegrainian telescope with a primary mirror diameter of 23 cm and a scanning mirror, which is spinning about a slightly tilted axis which itself is slowly rotated in a perpendicular direction to provide the second scan-dimension. With a typical distance of 0.5m between the scanning mirror and the object plane, the field of view covers several 100cm². While the fast mirror axis spins with about 660RPM, the slow axis turns with at least 1 deg. per second and the data acquisition samples about 40000 points for each THz-image. Single-pixel detectors are used; the frame acquisition time is below 10 s. The development of a video-rate multi-pixel imager with up to 32 sub-harmonic mixers as detectors is in progress.

This paper describes the design and performance of a ruggedized passive Terahertz imager, the frequency of operation is a 40GHz band centred around 250GHz. This system has been specifically targeted at vehicle mounted operation, outdoors in extreme environments. The unit incorporates temperature stabilization along with an anti-vibration chassis and is sealed to allow it to be used in a dusty environment. Within the system, a 250GHz heterodyne detector array is mated with optics and scanner to allow real time imaging out to 100 meters. First applications are envisaged to be stand-off, person borne IED detection to 30 meters but the unique properties in this frequency band present other potential uses such as seeing through smoke and fog. The possibility for use as a landing aid is discussed. A detailed description of the system design and video examples of typical imaging output will be presented.

We report experimental results on recently developed MEMS-based, uncooled THz detectors and imaging applications for linear focal plane arrays constructed from them. The detector incorporates a broadband micro-antenna coupled to an impedance-matched microbridge. Micro-antennas were fabricated having cut-on frequencies of 500GHz, 650GHz, and 1.5THz, each with bandwidth of several hundred GHz. Sensitivity and frequency response of the detectors is predicted to be ~6pW/√Hz (with backplane) and 7kHz, respectively, and supporting measurements of the first devices will be presented. Fully integrated 1x8 linear focal plane arrays have been assembled and will be used in on-going imaging demonstrations.

A method, suggested by us earlier for identification of materials with close spectra in terahertz range of frequencies and based on the analysis of medium response spectral lines dynamics, is verified experimentally. The temporal dynamics of spectral lines allows to determine relaxation time of rotational transitions as well. A question about measurement time, that is sufficient for determining of material response characteristic time, is discussed. To demonstrate the efficiency of proposed method, we treat the response of soap and chocolate under the action of terahertz pulse with a few cycles. Our investigation shows that it is possible to identify these materials with high probability.

In this paper, electron transport properties of terahertz (THz) step well quantum cascade laser structures are analyzed. These types of structures can allow for the radiative and LO-phonon transitions to be placed within the same well. Under such an arrangement there are three main energy levels, where the transition from the upper state to the middle state is at the THz radiative spacing and the transition from the middle state to the lower state is at or near the LOphonon energy (~ 36 meV in GaAs). The middle state (upper phonon or lower lasing state) is a single energy state, contrasting to previous LO-phonon based quantum cascade laser (QCL) designs that have doublet states. By having vertical radiative and LO-phonon transitions within the same well, it is possible for these types of structures to yield high oscillator strengths, which can lead to increased gain in the active region provided the upper state lifetime and injection efficiency are maintained. The step in the well allows for high injection efficiency due to the spatial separation of the wavefunctions. Monte Carlo simulations are used to analyze the structure in order to investigate these properties. Subband populations, electron temperatures, gain, and current density are extracted from the simulations. Comparisons are made to other existing conventional square well LO-phonon based QCLs. Our analysis indicates that these types of structures should be comparable to other design approaches and that step well injectors can be used to increase the injection efficiency for THz QCLs.

We have designed a terahertz imaging system, built with electronic components and operating at a single tunable frequency. The system scans in hybrid mode, combining coarse mechanical positioning with a fine scan produced by perturbing the beam with a system of opaque masks, placed into the collimated beam. The mask set is based on a modified Hadamard design, which aims at minimizing the loss of power and noise effects. The image acquisition is performed in transmission mode, with the sample placed at the focal plane. We present several imaging results obtained using our technique.

Excitation of electrically biased photoconductive switches with femtosecond optical pulses is a well established method of generating wide bandwidth (0.5 to 2.5 THz) pulses of terahertz frequency radiation. This method of pulse generation draws energy from the bias power supply to accelerate optically injected charge carriers, giving rise to a current pulse that can radiate into free space; nevertheless, the output power is limited by both poor coupling of the radiation out of the substrate and by charge carrier screening. By optimizing the electrode structures and illumination geometries it may be possible to address both of these limitations. To explore this idea, we have experimentally studied illumination with coherent array of two sources spaced by less than a wavelength and operated near saturation in a semi-insulating GaAs source. By illuminating with an array of N=2 spots, we demonstrate a doubling in output terahertz power with no increase in input optical power. This result is consistent with results that have been shown for optical excitation that has been stretched along the anode using cylindrical lenses; however, the array of two spots permits steering the beam by adjusting the relative time of arrival of the two exciting pulses. Experimental evidence of this beam steering is presented. With proper electrode design, this approach may ultimately enable an N-fold increase in output power with an array of N spots, the formation of narrow beams, and the adjustment of beam direction by control of the relative time of arrival of the exciting optical pulses.