This PDF file contains the front matter associated with SPIE Proceedings Volume 7215, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

We have fabricated and characterized plasmonic terahertz detectors that integrate a voltage controlled planar barrier with a grating gated GaAs/AlGaAs high electron mobility transistor. These detectors exhibit a narrowband, tunable plasmonic response. Substantially increased responsivity is achieved by introducing an independently biased, narrow gate that produces a lateral potential barrier adjacent to the drain when biased to pinch-off. DC electrical characterization in conjunction with bias-dependent terahertz responsivity and time constant measurements indicate that a hot electron bolometric effect is the dominant response mechanism over a broad range of experimental conditions. The temperature dependence of the bolometric response is consistent with the energy relaxation time and absorption coefficient of a 2DEG. Rectification resulting from non-linear current-voltage characteristics also appears to contribute to the response. Additionally, we have begun investigating the operation of this device with the full grating gate biased to pinch-off to produce many detection elements in series.

Integration of THz quantum cascade lasers (QCLs) with single-mode 75 μm x 37 μm rectangular waveguide components, including horn antennas, couplers, and bends, for operation at 3 THz has been designed and fabricated using thick gold micromachining. Measurements on the isolated waveguide components exhibit fairly low loss and integration with THz QCLs has been demonstrated. This technology offers the potential for realizing miniature integrated systems operating in the 3 THz frequency range.

We report on multi-channel detection of ultrashort THz pulses by a linear array of 16 photoconductive dipole antennas. The dipole antennas built on low-temperature grown GaAs are excited by a line focus of fs-pulses. By the parallel detection of a complete line of ultrashort THz pulses, the measurement speed of THz ultrashort pulse time domain systems can be accelerated by an order of magnitude. For demonstration, the THz beam profile along the line detector is determined, and its spectral dependence of the electric field distribution is compared and verified by wave-optical simulations.

We report on the first THz balanced mixer/upconverter using a Schottky diode MMIC chip. Using an optically pumped laser at 1562 GHz as an LO source with a coupled power of about 1 mW, and 1 mW input at an IF frequency of 10 GHz, we obtained a sideband output power of 23 uW (sum of two sidebands). As a mixer, at an LO of 1621 GHz, we obtain a conversion loss of 12.4 dB DSB and a noise temperature of 5600 K DSB. Response is believed to be similar over a band 1250-1650 GHz. New diodes have been designed for easier application as mixers up through 3 THz, and a new wafer run is in process.

Terahertz detection using excitations of plasmon modes offers a high-speed, high resolution, and frequency-selective alternative to existing technology. Plasmons in high mobility quantum well two-dimensional electron gas (2DEG) systems can couple to radiation when either the channel carrier density, or the incident radiation, is spatially modulated with appropriate periodicity. Grating-gated terahertz detectors having a voltage tunable frequency response have been developed based on this principle. A continuous wave THz photomixer was used to characterize the resonant absorption in such devices. At the fundamental 2DEG plasmon frequency, defined by the grating and the quantum well carrier density, a 20% change in transmission was observed. As the resonance is tuned from the 'natural' plasmon frequency through application of a gate bias, it shifts as expected, but the transmission change drops to only a few percent.

We present the first characterization of a simple subwavelength-diameter plastic wire by using wideband terahertz timedomain spectroscopy. The propagation characteristics including waveguide dispersion, group velocity, and attenuation constant of various plastic wires with different diameters and refractive indices are studied. The experimental results show the subwavelength plastic wire has extremely low waveguide dispersion and low attenuation constant (<0.01cm^-1) at its THz transmission band due to much reduced fractional power delivered inside the lossy core, which is consistent with the theoretical calculations. With the large evanescent-fields, the subwavelength plastic wire has capability to integrate with micro-fluid channel for sensitive bio-sensing applications.

Coherent terahertz radar systems, using CO₂ laser-pumped molecular lasers have been used during the past decade for radar scale modeling applications, as well as proof-of-principle demonstrations of remote detection of concealed weapons. The presentation will consider the potential for replacement of molecular laser sources by quantum cascade lasers. While the temporal and spatial characteristics of current THz QCLs limit their applicability, rapid progress is being made in resolving these issues. Specifications for satisfying the requirements of coherent short-range THz radars will be reviewed and the feasibility of incorporating existing QCL devices into such systems will be described.

We developed an active gas sensing system with a highly- sensitive sub-terahertz (THz) wave receiver consisting of a superconductor-insulator-superconductor mixer and a photonics-based THz-wave local oscillator. Continuous monitoring of the gas contents in the gas cell was successfully carried out with the developed system. The high sensitivity of the developed THz-wave receiver makes it possible to extend the detection range up to 30 m.

Liquid water is a very strong absorber in the THz frequency range. We have set-up a unique germanium laser spectrometer consisting of a Ge:Be laser, tunable from 1 to 4 THz, and a sensitive Ge photoconductor detector. The spectrometer uses a measurement scheme alternating sample and reference signal while placed in an environmentally controlled housing for high stability of temperature and humidity. The laser system leads to a very small statistical error in the absolute absorption coefficient (400-500 cm^-1) of less than 0.1% corresponding to 0.3 cm^-1 while systematic errors due to filling of the sample cells become dominant. The high accuracy allows us to systematically investigate the effects of different solvates on water dynamics. Even a single point mutation in a protein can be measured in the THz absorption coefficient in the spectral range from 2 to 3 THz. The system has been recently used to study various solvates in liquid water like sugars and prototype proteins in aqueous buffer solutions in dependence of temperature, pH values, and denaturants. These studies are now augmented by time-resolved measurements using THz time-domain spectroscopy to analyze the kinetics of protein folding. We also discuss other THz sources and detection methods including the investigation of coherent synchrotron radiation at the synchrotron ANKA in Karlsruhe.

We report on the realization of two active fully electronic THz cameras operating at different frequencies (645 GHz and 300 GHz) and room temperature. Active illumination together with the frequency modulation continuous wave approach allows us to implement unique features, such as phase-sensitive detection, working-distance selection and the suppression of spurious reflections. With both systems we are able to acquire images with more than 50000 pixels (phase and amplitude) in 9 seconds. The dynamic range exceeds 35 dB and we achieve a subwavelength depth resolution due to the measurement of the phase. The typical object distance is about 50-100 cm and the image size is on the order of hundreds of cm². With frequency modulation of the source we are furthermore able to detect the position form objects up to an accuracy of a few mm.

Terahertz (THz) spectroscopy is a promising technique for the stand-off detection and characterization of hidden objects. The THz band is particularly well suited firstly because THz radiation penetrates many dielectrics like clothes and secondly because many potentially hazardous substances have characteristic signatures in the THz spectral region. In order to utilize the full potential of THz radiation for detecting possible hazards and recognizing characteristic signatures, disturbing influences must be accounted for. We have performed experiments and simulations in order to investigate the limits of terahertz stand-off detection. A special emphasis is paid on humidity in ambient air and properties of the sample like surface roughness, alignment and interfaces. Water vapor absorption strongly affects the THz spectra. Since the absorption lines are strong and narrow, the calculation must be precise. We have checked various models well-known in meteorology covering the infrared and the microwave region of the electromagnetic spectrum. By extending the models into the THz region, an accurate description of the measured spectral absorption is achieved. In our studies transmission spectra for different substances were tested. In a reflection scheme metallized sandpaper of various grit sizes was used to determine the influences of different surface properties. Further measurements were performed for different tilt angles to analyze the influence of the surface roughness. We are currently creating a look-up table to show which parts of the THz spectrum can be used for THz stand-off detection.

Continuous wave terahertz imaging has the potential to offer a safe, non-invasive medical imaging modality for detecting different types of human cancers. The aim of this study was to identify intrinsic biomarkers for non-melanoma skin cancer and their absorption frequencies. Knowledge of these frequencies is a prerequisite for the optimal development of a continuous wave terahertz imaging system for detecting different types of skin cancers. The absorption characteristics of skin constituents were studied between 20 and 100 cm^-1 (0.6 THz - 3 THz). Terahertz radiation is highly absorbed by water. Thus, the high water content of human tissue necessitates a reflection based imaging modality. To demonstrate a reflection based, high resolution, terahertz imaging system, a prototype imaging system was constructed at 1.56 THz. The system resolution was determined to be 0.5 mm and the system signal to noise ratio was found to be 70 dB. Data from the terahertz spectroscopy experiments and reflection based terahertz images at 1.56 THz are presented.

We report on imaging at terahertz frequencies using a 3x5 Si MOSFET focal-plane array (FPA) processed by a 0.25-μm CMOS technology. Each pixel of the FPA consists of a 645-GHz patch antenna coupled to a FET detector and a 43-dB voltage amplifier with a 1.6-MHz bandwidth. We achieve a typical single-pixel responsivity of 80 kV/W and a noise-equivalent power (NEP) of 300 pW/√Hz at 30-kHz. The performance data of these all-CMOS devices pave the way for the realization of broad-band THz detectors and FPAs for video-rate active imaging on the basis of established low-cost and integration-friendly CMOS technology.

The U.S. Army Research Office (ARO) and the U.S. Army Edgewood Chemical Biological Center (ECBC) jointly lead and support novel research programs that are advancing the state-of-the-art in nanoelectronic engineering in application areas that have relevance to national defense and security. One fundamental research area that is presently being emphasized by ARO and ECBC is the exploratory investigation of new bio-molecular architectural concepts that can be used to achieve rapid, reagent-less detection and discrimination of biological warfare (BW) agents, through the control of multi-photon and multi-wavelength processes at the nanoscale. This paper will overview an ARO/ECBC led multidisciplinary research program presently under the support of the U.S. Defense Threat Reduction Agency (DTRA) that seeks to develop new devices and nanoelectronic architectures that are effective for extracting THz signatures from target bio-molecules. Here, emphasis will be placed on the new nanosensor concepts and THz/Optical measurement methodologies for spectral-based sequencing/identification of genetic molecules.

The Heterodyne Instrument for the Far-Infrared (HIFI) is one of three instruments to be launched aboard the Herschel Space Observatory (HSO) in 2009. HIFI will provide unprecedented spectral sensitivity and resolution between 490-1250 GHz and 1410-1910 GHz. In this paper, we report on the analysis of electrical standing waves that are present between the hot electron bolometer (HEB) heterodyne mixing element and the first low noise amplifier in the HIFI instrument. We show that the standing wave shape is not a standard sinusoid and difficult to remove from the resulting spectrum using standard fitting methods. We present a method to remove the standing waves based on data taken during the HIFI instrument level test, and anticipate the use of a similar calibration procedure in actual flight. Using the standing wave profile we obtain direct evidence of the complex IF output impedance of the HEB mixer.

Compact Quantum Cascade Laser waveguides have been analyzed using the full-vectorial finite element method. Modal intensity profiles, detailed power confinements and loss factors have been characterized for waveguides based on GaSb/AlGaSb multiple quantum well structures. Variations in these key parameters were also further investigated whilst varying the semiconductor doping concentration. Higher order modes having a low propagation loss were also shown.

The advent of terahertz (THz) spectroscopy and imaging has motivated the investigation of waveguides structures that are appropriate for use at THz frequencies. Currently, most spectroscopy systems have no guiding mechanism and therefore suffer spherical spreading loss. The ideal waveguide would eliminate these spreading losses, have low attenuation and dispersion, and high field confinement. Effective terahertz spectroscopy also requires especially large bandwidths to identify spectral signatures; therefore single-mode propagation is necessary to transmit broadband pulses. Single-mode waveguides can be achieved by utilizing the fundamental HEM₁₁ mode, which has no cutoff frequency, or by suppressing higher order modes. In this work, we present cylindrical, hollow-core, dielectric waveguides for the G-band (140-220 GHz). We will show propagation characteristics obtained by theoretical analysis, computer simulation, and experiments conducted using a vector network analyzer (VNA) with a cylindrical horn attachment. The analytical model, in particular, will provide an additional capability to quickly predict waveguide behavior for a variety of applications. This complete model will predict both the number of supported and propagating modes and the means to source them. We will show that the number of supported modes depends primarily on the thickness and dielectric constant of the outer layer. By choosing proper dimensions and materials, single-mode waveguides can potentially be designed and realized to achieve all of the aforementioned terahertz spectroscopy requirements.

Special approaches unique to the waveband are required for the modelling of terahertz optical systems. Ray tracing is inadequate because in typical instruments the propagating beams are not very many wavelengths in diameter and a "quasi-optical" approach is required in which Fresnel diffraction effects can be efficiently and accurately simulated. Typically, it is also necessary to be able to simulate the coupling of quasi-optical beams to feed antenna structures to predict optical performance. In many systems the beams can be considered to be coherent and their propagation through a beam guide consisting of re-focussing elements can be efficiently modelled using modal analysis, especially useful for quick design purposes, beam control and optimisation. This modal approach has been extended to allow for aberrations and truncation particularly relevant in compact mirror based systems. At the same time physical optics, although computationally intensive, is also a useful tool when detailed analysis is required, particularly for providing crosspolarisation information and high accuracy far-field beam patterns from large reflecting antennas, for example. However, modal analysis in general is a very powerful tool, which enables one also to understand issues associated with throughput when partially coherent systems are being considered. This is important for the efficient optical modelling of large arrays systems now being developed for next generation astronomical instrumentation. In the presentation, we will discuss these issues and present examples from real instrumentation. We also summarise our continuing work on the development of computationally efficient modelling tools for fast quasi-optical design and analysis.

The development of active THz cameras with the potential for video-rate operation is an emerging and exciting research field. With our currently realized 645 GHz system we achieve scan rates of a few seconds with a one-pixel heterodyne detector and two-dimensional fast rotational scanning. The active illumination allows to resolve the object topography with subwavelength resolution. Within the next evolution step we will realize an active 812 GHz system incorporating a 1x32-pixel heterodyne detector array with one-dimensional scanning. This will allow video-frame-rates for images (amplitude and phase) with approximately 2000 pixels. But for large fields of view the quasioptical system must be optimized to minimize the aberrations inherent in all optical systems. We show, with the use of the optical software package Zemax, how to design, simulate and optimize such quasioptical systems for the one-dimensional 1x32-pixel heterodyne detector array. The resulting quasioptical system is diffraction-limited over the field of view (20 cm x 30 cm) at the design working distance of 4 m and has an adjustable focus optics for distances from 2 m up to 6 m.