This PDF file contains the front matter associated with SPIE Proceedings Volume 7671, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

This paper reviews recent advances in emission and detection of terahertz (THz) radiation utilizing two-dimensional plasmons in semiconductor heterostructures and their possible sensing and spectroscopic applications. The device is introduced as a light source into a Fourier-transform THz spectrometer. Water-vapor absorption lines as well as fingerprints of honey and maple syrup of sugar-group materials were successfully observed. Absolute sensitivity characteristics and detection polarization are also presented, showing the possibility of new highly efficient THz detectors.

Superconducting hot electron bolometer (HEB) mixers are a competitive alternative to other technologies in the terahertz frequency range because of their ultrawide bandwidth, high conversion gain, and low intrinsic noise level. A process to fabricate stacked YBaCuO / PrBaCuO ultra-thin films (in the 15 to 40 nm range) etched to form 0.4 μm × 0.4 μm constrictions, elaborated on MgO (100) substrates, has been previously described. HEB structures were fabricated on such stacks, covered by log-periodic planar gold antennas, aiming at spanning the 0.9 to 7 THz range. Ageing effects were observed, however, with the consequence of increased electrical resistance, significant degradation of the regular bolometric response, so preventing HEB mixing action. Several measures have been attempted to address these problems, mainly by considering the embedding technological issues related to the YBaCuO constriction electrical coupling to the antenna and the intermediate frequency (IF) circuitry. For this purpose, the YBaCuO impedance was analyzed, and mismatch to antenna and IF strip was considered. Besides, THz antenna simulations were performed and validated against experiments on scaled models at GHz frequencies. Electromagnetic coupling to the incoming radiation was also studied, including crosstalk between neighbor antennas forming a linear imaging array.

We previously reported resonant photovoltaic terahertz detection via bulk plasmons in GaAs field-effect-transistors (FETs). Here, we introduce a device model which incorporates the microscopic dynamics of terahertz-field-driven electrons in the FET channel, resonant excitation of three dimensional (bulk) plasmons, and self-mixing theory of Lisauskas and Pfeiffer. The resulting model can simulate our experimental results and implies a bulk plasmon-assisted terahertz self-mixing process occurs in the FET-based terahertz detectors. The model also suggests three factors are important to improving the device performance – power coupling efficiency, self-mixing efficiency, and resonance with bulk plasmons.

A novel approach for submillimeter-wave heterodyne imaging arrays is presented in this paper. By utilizing diverse technologies such as GaAs membrane based terahertz diodes, wafer bonding, bulk Si micromachining, micro-lens optics, and CMOS 3-D chip architectures, a super-compact low-mass submillimeter-wave imaging array is envisioned. A fourwafer based silicon block for a working W-band power amplifier MMIC is demonstrated. This module drastically reduces mass and volume associated with metal block implementations without sacrificing performance. A path towards super compact array receivers in the 500-600 GHz range is described in detail.

The possibility of a compact source of coherent terahertz radiation is being realized through the development of quantum cascade lasers (QCL's). These lasers consist of a semiconducting heterostructure active region and an internal waveguide that make intraband lasing transitions possible. The use of terahertz QCL's in promising applications such as medical imaging, defense, and security is currently limited by low output laser power. Systematic optimization of the QCL's waveguide reduces mode losses, improves confinement, and increases output power. Waveguide optimization is especially important for lasers operating at low terahertz frequencies where semi-insulating surface-plasmon waveguide performance degrades significantly. Prediction codes have been developed that systematically optimize semi-insulating surface-plasmon waveguides. The methods and results of these optimizations will be presented for a full suite of terahertz QCL waveguides at different frequencies. The use of the optimization code to investigate graded-doping waveguide structures will also be presented.

Research into absorption spectra is useful for detecting chemicals in the field. Each molecule absorbs a set of specific frequencies, which are dependent on the molecule's structure. While theoretical models are available for predicting the absorption frequencies of a particular molecule, experimental measurements are a more reliable method of determining a molecule's actual absorption behavior. The goal of this research is to explore chemical markers (absorption frequencies) that can be used to identify highly energetic molecules of interest to the remote sensing community. Particular attention was paid to the frequency ranges located within the terahertz transmission windows of the atmosphere. In addition, theoretical derivations, with the purpose of calculating the detection limits of such chemicals, will also be presented.

We investigated resonance spectroscopic features from several widely used explosives materials including RDX and PETN in the low THz range with the goal of understanding the mechanism of interaction between radiation and material in the form of solid films, gels and dilute solutions (suspensions). FTIR spectroscopy was used to measure spectra in transmission and reflection modes. We demonstrated that very small amount of material with a simple sample preparation technique can be used still providing very accurate results. Spectral features are specific not only for main ingredients but for modifications with different plasticizers. The consistency of results for different amount of material was observed. Computational modeling confirmed the lowest frequency modes.

We report on arrays of THz-emitters based on n-i-pn-i-p-superlattice photomixers for imaging and spectroscopic applications. The output power of a n-i-pn-i-p superlattice photomixers recently has reached nearly 1 μW at 1 THz with a broadband antenna. There are no fundamental physical limitations at this stage for further improvement. Tunable continuous wave (CW) THz-sources for imaging and spectroscopy are highly desired tools for security and environmental applications. In particular, most stand-off imaging applications require a rather high THz power to allow for a sufficient dynamic range, and a narrow illumination spot size for high spatial resolution. Both goals can be reached by using an array of mutually coherent photomixers. We have simulated beam patterns for an arbitrary number of mutually coherent single sources with respect to a small beam size and high peak intensity. Here, we confirm the simulations experimentally by an array of 4 sources with a 4 inch THz optics. The beam profile is measured in the target plane at a stand-off distance of 4.2 m. As a result, the beam diameter is reduced by a factor of 6 and the peak intensity is enhanced by a factor of close to (4)² = 16, in excellent agreement with our simulations. Such an arrangement allows not only for high resolution stand-off imaging but also for spectroscopic investigations at stand-off distances. The THz frequency can be tuned over more than a decade (i.e. 0.1 to 2.5 THz) by tuning the wavelength of the mixing lasers. The spectral linewidth of the THz sources is only limited by the linewidths of the mixing lasers and can be made extremely narrow. A straightforward demonstration is achieved by water vapor spectroscopy in laboratory air with a single source.

This paper discusses the effect of periodic roughness and surface defects on the electromagnetic scattering of terahertz waves from cylindrical objects. The cylinders, possessing periodic roughness imparted during their fabrication process, had average roughness values ranging from approximately 0.1 μm - 0.50 μm. Metallic cylinders were fabricated from lathe-turned aluminum rods and dielectric cylinders were fabricated using a rapid prototype technique (stereolithography). The scattering behavior of the rough cylinders was measured in 160 GHz and 350 GHz compact radar ranges. In addition, the effect of seams and grooves on the scattering behavior of cylinders will also be presented.

Single resonance chemical remote sensing, such as Fourier-transform infrared spectroscopy, has limited recognition specificity because of atmospheric pressure broadening. Active interrogation techniques promise much greater chemical recognition that can overcome the limits imposed by atmospheric pressure broadening. Here we introduce infrared - terahertz (IR/THz) double resonance spectroscopy as an active means of chemical remote sensing that retains recognition specificity through rare, molecule-unique coincidences between IR molecular absorption and a line-tunable CO₂ excitation laser. The laser-induced double resonance is observed as a modulated THz spectrum monitored by a THz transceiver. As an example, our analysis indicates that a 1 ppm cloud of CH₃F 100 m thick can be detected at distances up to 1 km using this technique.

The method of THz spectrum dynamics analysis (SDA-method) is used for identification of compound media and detection of their components. The algorithm of identification is based on the SVD-method (method of Singular Value Decomposition) for reconstruction of the pulse shape by one set of measurements of integral characteristics from medium response during time intervals instead of the measurement of an instant medium response and on sliding window method for obtaining the spectrogram - dynamics of Fourier spectrum. It allows us to trace the dynamics of many spectral lines in one set of measurements and to obtain the full information about the spectrum dynamics of the signal, passed through the medium, even for measurements that are made over a short-term interval (less than 20 ps). We consider the examples imitating the mixture of explosive with harmless substances in different ratios. With this aim we analyze a sum of two signals, passed through both neutral material and explosive. Our investigations have shown that spectrogram of the signals sum differs clearness from spectrogram, obtained for neutral substance. Essential differences one can see in dynamics of spectral lines for both cases of signals. It is possible to detect the presence of additional substances in the mixture with neutral material even if the amplitude of the signal from explosive is 25 times less than the amplitude of the signal from neutral material. Therefore, the method allows for detecting and identifying the substance in compound media with high probability and can be very effective for defense and security applications. We compare as well the efficiency of our approach for identification of explosive with the efficiency of using the autocorrelation function and show that the autocorrelation function has less resolvability in comparison with the SDA-method.

The underlying THz vibrational spectrum of explosives and related threat materials consists of a highly detailed fingerprint due to the interaction of internal and external vibrations in the crystalline environment. However, the underlying spectrum has been difficult to resolve, even at cryogenic temperatures, because of line broadening processes. In this paper we use the technique of waveguide terahertz time-domain spectroscopy (THz-TDS) to measure the underlying THz vibrational spectrum of three threat-related materials: 2,4-dintrotoleuene (2,4-DNT), 4-aminodintrotoluene (4A-DNT), and pentaerythritol dinitrotoluene (PETN). These materials are characterized as polycrystalline layers in the 50 micron gap of a metal parallel plate waveguide. For each material, we show that waveguide THz-TDS measurements at cryogenic temperature resolve, at least partially, the underlying THz vibrational spectrum. We suggest that these narrow-line waveguide THz-TDS measurements may be used as rigorous input for theoretical computations to predict THz vibrational spectra.

Novel device called the Ballistic Deflection Transistor (BDT) is presented. BDT is a unique planar device that possesses both a positive and a negative transconductance region and is capable of operating into the THz frequency regime at the room temperature. BDT is based upon an electron steering and a ballistic deflection effect. Modeling and experimental measurements have indicated that the transconductance of the device increases with applied drain-source voltage. The differential mode of operation provides two drain outputs, which depending on the gate bias, are either complementary or non-complementary. The latter facilitates a wide variety of circuit design techniques. The extremely low gate capacitance of the BDT planar structure predicts THz performance. We present measured results from the fabrication of a BDT NAND gate and other BDT gate structures. The measured results in the quasi-ballistic regime are further supported by an empirical model generated from a fabricated BDT output response. Future plans to facilitate large-scale integration are also discussed. Our Monte Carlo analysis reports on the effect of different geometry parameters on the transfer characteristics. The strength of the gate control in the InGaAs channel is analyzed. We propose different models for the surface charge density to explain the observed experimental measurements. Finally, we present our time-domain spectroscopy studies used to successfully demonstrate a THz response of ballistic nanodevices at room temperature, excited by picosecond electrical pulses.

The standalone, portable Terahertz (THz) Imaging Profiler Array (TIPA) based on an Offner Relay design has been constructed as a THz beam profiler and multispectral imager. It integrates a solid-state detector technology (Schottky Diodes) that can be configured in an array to cover the frequency range from 0.60 to 0.90 THz. The reconfigurable 16 element Schottky diode detector array is utilized along with imaging and scanning mirror modules and system control hardware and software to produce high spatial or temporal beam profiles of THz beams. Images of THz source profiles are presented along with THz images of relevant targets. Potential applications are discussed.

A portable video rate time-domain terahertz (TD-THz) reflection line-scanner suitable for aerospace destructive examination (NDE) and security inspection is described. The imager scans a line 6 inches wide and collects a TD-THz cross-sectional "B-scan" of the sub-surface structure at rates up to 30 Hz. The imager is hand-held. By rolling the scanner over surface, a radiographic two dimensional "C-Scan" image can be stitched together from the individual lines at a rate of 1-4 inches per second (depending on desired resolution). The case is 8.7 in. wide (12.9 in. with wheels), 12.5 in. long, and 7.9 in. high. The weight is approximately 11 lbs. Example images taken with the scanner of radome THz NDE are shown.

When a terahertz laser generator irradiates a person, its skin gives a thermal and thermoelastic response. Solving the partial differential equations of heat conduction propagation in cylindrical coordinates and the dynamical equations of thermoelasticity with the help of special mathematical functions and laplace transformation, I obtained two different functions in dependence of one spatial coordinate and time. These two functions can be used to perform an analysis of the skin's response, and thus will help to differentiate between unhealthy skin tissue and healthy one.

In this paper, designs of ultra-fast all-optical based Terahertz-optical-asymmetric-demultiplexer (TOAD)-based devices are reported. Using TOAD switches, adders/subtracters units are demonstrated. The high speed is achieved due to the use of the nonlinear optical materials and the nonbinary modified signed-digit (MSD) number representation. The proposed all-optical circuits are compared in terms of numbers TOAD switches, optical amplifiers and wavelength converters.

We have suggested a wide range of real-life applications using novel terahertz imaging techniques. A high-resolution terahertz tomography was demonstrated by ultra short terahertz pulses using optical fiber and a nonlinear organic crystal. We also describe a non-destructive inspection system that can monitor the soot distribution in the ceramic filter using millimeter-to-terahertz wave computed tomography. Further we report on the thickness measurement of very thin films using high-sensitivity metal mesh filter. These techniques are directly applicable to the non-destructive testing in industries.

In this paper, we present a design for a widely tunable solid-state optically and electrically pumped THz laser based on the Smith-Purcell free-electron laser. In the free-electron laser, an energetic electron beam pumps a metallic grating to generate surface plasmons. Our solid-state optically pumped design consists of a thin layer of dielectic, such as SiN_x, sandwiched between a corrugated structure and a thin metal or semiconductor layer. The lower layer is for current streaming, and replaces the electron beam in the original design. The upper layer consists of one micro-grating for coupling the electromagnetic field in, another for coupling out, and a nano-grating for coupling with the current in the lower layer for electromagnetic field generation. The surface plasmon waves generated from the upper layer by an external electromagnetic field, and the lower layer by the applied current, are coupled. Emission enhancement occurs when the plasmonic waves in both layers are resonantly coupled.

A low loss, hollow, flexible waveguide was designed and fabricated for the transmission of terahertz radiation. Polycarbonate and glass tubing was chosen as the base material of the waveguide. The inner surface of the dielectric tube was coated with silver using a wet chemistry method. The waveguides were fabricated in several diameters namely, 2.4 mm, 3.2 mm, 3.8 mm, and 4.6 mm. The attenuation coefficients for the waveguides were measured at a wavelength of 215 μm using an optically pumped terahertz laser. The free space Gaussian mode from the terahertz laser source was coupled into the lowest loss TE₁₁ mode inside the waveguide. The mode profiles of the output beam from the waveguides were shown. A loss of 3.5 dB/m was measured for the silver coated dielectric tube of diameter 4.6 mm.

We report here the performance improvement of frequency doublers using substrate transfer technique, the method by which the diodes/MMIC circuits made on GaAs substrate have been subsequently transferred on to a host substrate like Quartz and Aluminium Nitride. These host substrates have low loss and high thermal conductivity at mm-wave and sub millimeter wave frequencies. The substrate transfer technique on RAL doubler circuits designed at 160 GHz gives a conversion efficiency ~ 30% and 3 dB BW >15%, which is a significant performance improvement compared to the same diodes on GaAs substrate. The efficiency and bandwidth at a constant input power has been studied using doubler diodes of different anode areas and the results are presented in this paper. The measured data is compared to simulations, and the test results agree closely to predictions.

77 GHz passive millimeter wave (PMMW) imaging camera for the purpose of security is developed. In order to detect concealed objects in clothes without hindrance to flow of people at airport security checks, video rate imaging is realized using one-dimensional imaging sensor array of 25 elements and a flapping reflector. As receiving antennas, novel antipodal Fermi antenna (APFA) having required characteristics for passive imaging such as broad bandwidth to obtain enough power, axially symmetric directivity with 10dB beam width of 35 degrees to obtain optimum coupling with dielectric lens, narrow width geometry for high spatial resolution of imaging is used. Real-time calibration (RTC) technique is introduced to eliminate the drift of receiving circuits. Interpolation technique to improve the quality of image and marking software for screening of suspicious objects are also developed. High spatial resolution of 20 mm is obtained by using developed imaging camera.

Brownout, the loss of visibility caused by dust and debris introduced into the atmosphere by the downwash of a helicopter, currently represents a serious challenge to U.S. military operations in Iraq and Afghanistan, where it has been cited as a factor in the majority of helicopter accidents. Brownout not only reduces visibility, but can create visual illusions for the pilot and difficult conditions for crew beneath the aircraft. Terahertz imaging may provide one solution to this problem. Terahertz frequency radiation readily propagates through the dirt aerosols present in brownout, and therefore can provide an imaging capability to improve effective visibility for pilots, helping prevent the associated accidents. To properly model the success of such systems, it is necessary to determine the optical properties of such obscurants in the terahertz regime. This research attempts to empirically determine, and measure in the laboratory, the full complex index of refraction optical properties of dirt aerosols representative of brownout conditions. These properties are incorporated into the AFIT/CDE Laser Environmental Effects Definition and Reference (LEEDR) software, allowing this program to more accurately assess the propagation of terahertz radiation under brownout conditions than was done in the past with estimated optical properties.

YBa₂Cu₃O_6+x compounds are well known to exhibit superconducting properties for x > 0.5 and semiconducting properties for lower oxygen content. Superconducting YBCO was obtained commercially; the semiconducting material was deposited by sputtering at room temperature. In order to migrate from superconducting to uncooled semiconducting far-infrared bolometer technologies, we have first realized and compared the performance of 2 × 2 pixel arrays made from both materials deposited on MgO substrates. Pixels were in the shape of meanders, embedded in an area of about 1 mm². Pixel detectivity and thermal crosstalk were studied in the 1 Hz to 100 kHz modulation frequency range by using a 850 nm solid state laser. Secondly we have improved the geometry of semiconducting YBCO bolometers fabricated on silicon substrates, in order to match their impedance with the impedance of the antenna required for working in the THz range. First optical results are also presented, where both regular bolometric and pyroelectric responses are exhibited.

We report improvements in the scanning speed and standoff range of an ultra-wide bandwidth terahertz (THz) imaging radar for person-borne concealed object detection. Fast beam scanning of the single-transceiver radar is accomplished by rapidly deflecting a flat, light-weight subreflector in a confocal Gregorian optical geometry. With RF back-end improvements also implemented, the radar imaging rate has increased by a factor of about 30 compared to that achieved previously in a 4 m standoff prototype instrument. In addition, a new 100 cm diameter ellipsoidal aluminum reflector yields beam spot diameters of approximately 1 cm over a 50×50 cm field of view at a range of 25 m, although some aberrations are observed that probably arise from misaligned optics. Through-clothes images of concealed pipes at 25 m range, acquired in 5 seconds, are presented, and the impact of reduced signal-to-noise from an even faster frame rate is analyzed. These results inform the requirements for eventually achieving sub-second or video-rate THz radar imaging.

THz imaging is a very promising field rapidly growing in importance. This expanding field is at its early stage of development but already a large number of applications are foreseen. THz imaging promises to be a key technology in various fields, such as defense & security where it can be used to defeat camouflage. Based on its many years of experience in uncooled bolometers technology, INO has developed, assembled and characterized a prototype THz imager. The camera's 160 × 120 pixel array consists of pixels with a 52 μm pitch that have been optimized for the THz region. Custom camera electronics and an F/1 THz lens barrel complete the imager design. Real-time imaging at video rate of 30 frame/sec has been performed with a 3 THz quantum cascade laser set-up. THz images of numerous object-obscurant combinations are presented, proving the feasibility of video imaging in security screening applications.

The ability of terahertz and millimeter-wave imaging to detect suspicious hidden objects underneath or in luggage has led to increased interest in these techniques. Several approaches have been demonstrated in the past few years, amongst which active, all-electronic terahertz imaging has proven to be particularly adapted for safety and security applications. It combines a large dynamic range and the ability to perform range measurements with increased spatial resolution. At the French-German Research Institute of Saint Louis (ISL), we use an all-electronic 3D imaging system for a comprehensive study on various suspicious objects and cloth types. We demonstrate an enhanced detection capability for hidden suspicious objects if the range information is extracted and visualized in appropriate ways.

A novel terahertz source that is capable of emitting terahertz radiation based on a phase-matched optical rectification process pumped by 1550-nm fiber lasers is proposed. The whole device equivalently consists of many THz-emitter units connected along the direction of optical pulses propagation by optical chirp compensation components such as photonic crystal fibers and optical semiconductor amplifiers. In each THz-emitter unit, there are two sections adjacent to each other along the direction of the pumping optical pulse propagation. One of the sections is a segment of asymmetric dielectric planar waveguide in which a core-layer of GaAs is contacted with a substrate containing an epilayer of Al_xGa_1-xAs. The other section is a segment of corrugated waveguide with a GaAs second-order rectangular grating teeth layer above the same substrate containing an epilayer of Al_xGa_1-xAs. The terahertz waves could be generated in the non-grating section by a pulsed fiber laser based on an optical rectification process under the circumstance of phase matching by exploiting the waveguide mode dispersion. These terahertz waves then could be coupled into the air and the substrate transversely in the following grating section to form the terahertz radiation.