This PDF file contains the front matter associated with SPIE Proceedings Volume 6549, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

The U.S. Army Night Vision and Electronic Sensors Directorate (NVESD) and the U.S. Army Research Laboratory (ARL) have developed a terahertz-band imaging system performance model for detection and identification of concealed weaponry. The details of this MATLAB-based model which accounts for the effects of all critical sensor and display components, and for the effects of atmospheric attenuation, concealment material attenuation, and active illumination, were reported on at the 2005 SPIE Europe Security and Defence Symposium. The focus of this paper is to report on recent advances to the base model which have been designed to more realistically account for the dramatic impact that target and background orientation can have on target observability as related to specular and Lambertian reflections captured by an active-illumination-based imaging system. The advanced terahertz-band imaging system performance model now also accounts for target and background thermal emission, and has been recast into a user-friendly, Windows-executable tool. This advanced THz model has been developed in support of the Defense Advanced Research Project Agency's (DARPA) Terahertz Imaging Focal-Plane Technology (TIFT) program. This paper will describe the advanced THz model and its new radiometric sub-model in detail, and provide modeling and experimental results on target observability as a function of target and background orientation.

Transmittance spectra of solid and vapor samples of trinitrotoluene (TNT) in the spectral range 0.6 to 10 THz at resolutions up to 1 GHz are reported. Uniform solid samples of ~100 &mgr;m thickness gave stronger absorption and more resolved structure than previous studies. New absorption lines for TNT solid below 100 cm^-1 are reported. A heated 10 m multpass White cell was used for spectroscopy of the vapor. Strong absorption bands yield unexpectedly large absorption cross sections for the anticipated saturated vapor pressure at the cell temperature, leaving their assignment to TNT in doubt. These results indicate that path lengths exceeding 10 m and temperatures higher than 40 C, or significantly higher instrumental sensitivity, are needed for sensing of TNT vapor in the spectral range 0.6 to 10 THz.

A primary source of "clutter" in sub-millimeter wave and terahertz imagery used in security applications is the random reflections from clothing. In this paper, techniques for modeling and characterizing these reflections are described. This work is motivated and, in part, based on previous work done in support of imaging radar for remote sensing. A first order model of the response of a cloth covered object is described along with a method for performing measurements on draped cloth. The measurement method involves the simultaneous measurement of the sub-millimeter wave response of the cloth and the underlying drape of the cloth. A rigorous model of the scattering from draped cloth is developed and compared with results from the first order model. Conclusions regarding the suitability of the first order model for image simulation and performance predictions are stated.

Broadband terahertz time-domain spectroscopy (THz-TDS) has been shown to be a valuable technique for the detection and analysis of explosives. In this paper, we present recent work on the use of this technique to analyse two pure explosives, 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and 1,3-dinitrato-2,2-bis(nitratomethyl)propane (PETN) and three plastic explosives (Semtex, SX2 and Metabel). For each explosive, a clear and unique spectral absorption fingerprint is found, which can be attributed to oscillations of the molecular lattice. Temperature dependent terahertz absorption measurements of RDX show that the observed vibrational modes are anharmonic in nature. Terahertz spectra are compared with Raman spectra, and the similarities and differences between the spectra produced by the two techniques are discussed. A molecular mechanics calculation approach is used to calculate vibrational intensities and frequencies, and these results are compared with experimental results at both room temperature and 4 K.

We demonstrate the location and identification of delaminations and water intrusion in advanced composite materials used in ground based radome panels, shelters and towers using time domain terahertz imaging. Water has a very high absorption in the THz spectral region in comparison to polymer foam and fiberglass, so the method is very sensitive to water intrusion. The sub-picosecond near single cycle electromagnetic images enabled identification of delaminations of the fiberglass shell and foam. We report a novel time domain terahertz non-destructive evaluation control system, which is suitably portable for applications such as field examination of radome composites.

We report the realization of a hybrid system for stand-off THz reflectrometry measurements. The design combines the best of two worlds: the high radiation power of sub-THz micro-electronic emitters and the high sensitivity of coherent opto-electronic detection. Our system is based on a commercially available multiplied Gunn source with a cw output power of 0.6 mW at 0.65 THz. We combine it with electro-optic mixing with femtosecond light pulses in a ZnTe crystal. This scheme can be described as heterodyne detection with a Ti:sapphire fs-laser acting as local oscillator and therefore allows for phase-sensitive measurements. Example images of test objects are obtained with mechanical scanning optics and with measurement times per pixel as short as 10 ms. The test objects are placed at a distance of 1 m from the detector and also from the source. The results indicate diffraction-limited resolution. Different contrast mechanisms, based on absorption, scattering, and difference in optical thickness are employed. Our evaluation shows that it should be possible to realize a real-time multi-pixel detector with several hundreds of pixels and a dynamic range of at least two orders of magnitude in power.

This paper describes the design and performance of the U.S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate's (NVESD), active 0.640-THz imaging testbed, developed in support of the Defense Advanced Research Project Agency's (DARPA) Terahertz Imaging Focal-Plane Technology (TIFT) program. The laboratory measurements and standoff images were acquired during the development of a NVESD and Army Research Laboratory terahertz imaging performance model. The imaging testbed is based on a 12-inch-diameter Off-Axis Elliptical (OAE) mirror designed with one focal length at 1 m and the other at 10 m. This paper will describe the design considerations of the OAE-mirror, dual-capability, active imaging testbed, as well as measurement/imaging results used to further develop the model.

Experimental results of homodyne terahertz interferometric 2-D imaging of RDX are presented. Continuous waves at 0.25-0.6 THz are used to obtain images of a C-4 sample at several THz frequencies. The performance of an N element detector array is imitated by only one detector placed at N positions. The distance between the C-4 sample and the detector array is ~30 cm. By taking interferometric images at several THz frequencies RDX can be recognized by the spectral peak at 0.82 THz. Simulations of interferometric images of two point sources of spherical waves are presented. The terahertz interferometric imaging method can be used in defense and security applications to detect concealed weapons, explosives as well as chemical and biological agents.

Suicide bombers and hidden bombs or explosives have become serious threats especially for mass transportation. Until now there exists no established system which can be used against these threats. Therefore new technologies especially for stand-off detection of threats are required. Terahertz (THz) rays offer an alternative inspection method, which can cope with these new challenges. Major advantages of THz radiation as compared to other spectral regions are the possibility to penetrate through clothes and that THz radiation is not harmful for human health. In this report the design and results of a THz stand-off detection system will be presented. The sensor is based on active illumination of the object and sensitive heterodyne detection of reflected and backscattered radiation. The system operates at about 0.8 THz. A THz laser is used for illumination and a superconducting hot-electron bolometric mixer for detection. The local oscillator required for heterodyne detection is a multiplied microwave source. The optical system is designed to allow for stand-off detection at 20 m with a spatial resolution less than 2 cm.

Goodrich has been developing a high resolution, broad band spectrometer that operates in the Terahertz (THz) region of the spectrum with the intent of performing chemical detection. THz spectroscopy exploits rotational resonances for detection of gas phase compounds. High resolution THz spectroscopy can improve detection and identification through increased probability of detection and reduced false alarms. The Goodrich THz spectrometer is based upon CW photomixer technology in a heterodyne configuration. The current Goodrich design offers continuous tunability across a 0.1 THz to 1.2 THz frequency range. One of the unique aspects of the Goodrich spectrometer is laser system control that has demonstrated difference frequency line widths on the order of 1.5 MHz with stability measured over long time scales. Absolute frequency accuracy is of the order of 4 MHz. The spectrometer design enables high THz energy densities with narrow line widths tunable over a broad spectrum. The system has demonstrated SNR better than a cryogenically cooled hot electron bolometer. This capability allows the Goodrich system to accurately determine absorption signatures of multiple chemicals with exceptional performance. Goodrich has completed initial system testing and verified performance. Initial tests were completed to determine SNR of the heterodyne photomixer transceiver. System performance was also verified for laser line width, stability, and repeatability. The spectrometer was tested against various toxic industrial chemicals. Preliminary data for HCN, HCl, NH₃, and SO₂ is presented.

The THz wavelengths cover the frequency range of 0.1-10 THz or 30-3000 &mgr;m wavelength band. Currently, detection of THz radiation is carried out using either antenna-coupled semiconductor detectors or superconducting bolometers. Imaging of objects using these detection schemes requires complex scanning mechanisms which limits the applications involving real time imaging. For imaging applications it is desirable to employ focal plane arrays (FPAs) which leads to more compact systems. The FPAs based on photon detectors commonly used in infrared require cooling which becomes stringent as the detection extends to THz wavelengths. On the other hand, microbolometer FPAs using thermal detectors based on temperature change due to infrared absorption have a broad wavelength response and can be operated at room temperature. The advances of microbolometer technology allow real time imaging in the 7-13 &mgr;m wavelength range with relatively high sensitivity. However, their ability to detect THz radiation is relatively unknown. In this paper, imaging of a 3.4 THz (88 &mgr;m) laser beam using an uncooled microbolometer camera is described.

The SuperCOnducting Terahertz Imager (SCOTI) is a small Cassegrain-type telescope with a scanning secondary mirror designed for a frequency of 0.34 THz. It can map objects at a distance of 5 meter using a small array of superconducting bolometers. The resolution at the object area is about 1 cm. Using SCOTI purely passive images of interesting objects can be taken, thus opening a wide field of applications.

An integrated continuous-wave (CW) terahertz biosensor is proposed based on an edge-coupled terahertz photomixer source with guided-wave optical excitation scheme. In this device, two laser beams are guided inside an optical dielectric waveguide structure and being gradually absorbed by an overlying ultra-fast photoabsorbing layer, wherein a terahertz signal is generated due to photomixing phenomenon. The generated THz signal is guided by a coplanar-stripline (CPS) and is coupled to an integrated CPS resonator, which acts as a sample carrier and transducer. After interaction by bio-sample, terahertz wave is guided by a CPS line to a wide-band antenna and is detected by a THz power detector. Our performance analysis for the proposed CW terahertz biosensor supports the feasibility of the whole idea very well. The proposed device is attractive for system-on-a-chip terahertz sensors and spectrometers.

The dynamic range of the signal return from metals is a significant source of image interpretation difficulty. Techniques such as logarithmic image compression have been used to improve the recognition. Alternative techniques for improvement may be developed. This development depends in part on the ability to accurately model the surface reflective behavior including phase shifts introduced by the reflection. This work presents the results of an enhanced model development. Models of high frequency behavior in materials divide into regions such as non-relaxation region, relaxation region, optical absorption and plasma frequencies. In traditional infrared and longer wavelength imaging systems, optical absorption may play a role and it is generally assumed that the system operates in or very near the relaxation region defined as frequencies significantly greater than the reciprocal of the Boltzmann relaxation time. Though typical THz frequencies are below the relaxation time, they are not far enough below to be considered completely in the non-relaxation region. This introduces a number of issues atypical of imaging in either the RF or IR regime. Further realism is gained from the incorporation of plastic into the reflectivity and emissivity model. Empirical model validation is accomplished for selected materials.

Surface-wave propagation on coated or uncoated conducting wires, rods, or tubes has been shown to provide low attenuation, moderate field extent, low dispersion, and high power-handling at frequencies from 100 GHz to 1 THz. Typical conductors are copper, aluminum, or stainless steel. Uncoated conductors provide the lowest loss, while conductors coated with a thin layer of low-loss dielectric (such as Teflon, polystyrene, or polyethylene) have the smallest field extent. The guided mode is the TM₀₁, often referred to as the Sommerfeld mode. The properties of the guided wave were theoretically analyzed by King and Wiltse in 1962, and measured results were obtained at 105 and 140 GHz. In the last two years the work has been rediscovered, and now four different research groups have reported new results. While the earlier work was conducted in the search for long, low-loss transmission lines (100 meters to 1 kilometer), the current applications are for lengths about 1 meter long, as might be used in probes. The recent results will be summarized, and an optimized design will be presented, along with general curves of attenuation, field extent, and power handling capability from 100 GHz to 1 THz.

A study of the vibration characteristics of a deoxyguanosine molecule and a deoxyadenosine molecule bonded onto the surface of silicon is presented. The vibrations of the systems can be classified into five modes: nonlocal vibrational modes, local vibrational modes, quasi-local modes, backbone vibrational modes and molecular bond vibrational modes. The general separation of the molecular bond modes (i.e., which occur in the infrared region) and other vibrational modes (i.e., which occur in the far-infrared (Far-IR) region) is only weakly influenced by mounting the molecules onto the surface of silicon through linker molecules. The main influence of the binding of the molecule onto to the surface of a silicon substrate is the shifting of the vibrational modes towards the terahertz regime (i.e., ~ 100 cm^-1) and an associated increase of the number of these low frequency modes. Furthermore, the FAR-IR active vibrational regions (i.e., defined where there exists the strongest absorption peaks) are in the range of 300 cm^-1 to 1903 cm^-1 for a deoxyguanosine molecule and 500 cm^-1 to 1841 cm^-1 for a deoxyadenosine molecule, respectively. For frequencies below these Far-IR regions, the absorption intensity is small. However, the vibrations in this region are almost all nonlocal vibration modes which are important for the study of interaction between bases and for the development of sequence information of DNA molecules in terms of optical techniques.

It is well-known that many explosives have characteristic terahertz (THz) absorption features, and that THz waves can penetrate many dielectric materials. However security applications generally prohibit using THz technology for transmission measurements, either because of standoff distances, thick targets, or opaque targets (metals). As a result, we focus our attention on THz reflection spectroscopy. We have measured the THz reflectivity signature of RDX residues on smooth metal surfaces that contain about 0.4 mg of RDX. We discuss our efforts in detecting trace explosives in reflection as well as our recent results including THz spectroscopy of four explosives from 1 to 6 THz, and measurement of the absolute absorption cross-section of explosives.

This work presents the application of a basic unsupervised classification algorithm for the segmentation of indoor passive Terahertz images. The 30,000 pixel broadband images of a person with concealed weapons under clothing are taken at a range of 0.8-2m over a frequency range of 0.1-1.2THz using single-pixel row-based raster scanning. The spiral-antenna coupled 36x1x0.02&mgr;m Nb bridge cryogenic micro-bolometers are developed at NIST-Optoelectronics Division. The antenna is evaporated on a 250&mgr;m thick Si substrate with a 4mm diameter hyper-hemispherical Si lens. The NETD of the microbolometer is 125mK at an integration time of 30 ms. The background temperature calibration is performed with a known 25 pixel source above 330 K, and a measured background fluctuation of 200-500mK. Several weapons were concealed under different fabrics: cotton, polyester, windblocker jacket and thermal sweater. Measured temperature contrasts ranged from 0.5-1K for wrinkles in clothing to 5K for a zipper and 8K for the concealed weapon. In order to automate feature detection in the images, some image processing and pattern recognition techniques have been applied and the results are presented here. We show that even simple algorithms, that can potentially be performed in real time, are capable of differentiating between a metal and a dielectric object concealed under clothing. Additionally, we show that pre-processing can reveal low temperature contrast features, such as folds in clothing.

Solid-state organic compounds such as &agr;-lactose-monohydrate and biotin have been shown to have narrow and intense THz absorption features at room temperature. Interest in lineshapes in the THz region is justified not only for practical reasons, since they are of crucial importance to spectroscopy-based identification of materials, but also because of the information the line-widths contain about the solid-state physics of the materials. The line-width of THz absorption features (generally from lattice vibrations) in solids is excepted to be inversely proportional to the scattering time of optical phonons. The line-width of absorption features might thus have implications on the solid-state physics of the material, in particular, the interaction of phonons and the phonon density of states. We use a continuous wave THz photomixing system to obtain a high resolution spectrum of &agr;-lactose-mohohydrate and analyze two of its lowest-frequency absorption lines. For comparison we measure the transmission spectra of 5 chemically related saccharides: melecitose, trehalose, maltose, cellobiose, and raffinose. Since &agr;-lactose-monohydrate has a stronger and narrower absorption feature than any of its related saccharides, this comparison study is an important step in understanding the mechanism of THz radiation absorption by organic solids and what line-widths to expect in THz spectroscopy.

We report the experimental results of a comparison between free-standing Nb and NbN microbolometer bridges coupled to equiangular spiral antennas on Si substrates. Because of the difference in material resistivity, bolometer resistance and aspect ratio is varied independently. Room-temperature antenna patterns measured at 650 GHz with a backward-wave oscillator are presented, as are I-V curves at T = 300 K and at T = 4 K. At room temperature, zero-bias resistance and specific responsivity are examined, and at 4 K, normal-state resistance and saturation power are studied. Nb devices display significantly lower saturation powers than NbN devices whose dimensions have been adjusted to provide equal resistance. However, for both materials, the inferred thermal conductances are higher than predicted by the Wiedemann-Franz relation, by approximately a factor of ~2 for Nb and a factor of ~5 for NbN. In general, and especially for the room-temperature responsivity, the substantial spread in device parameters from device to device exceeds any systematic difference in performance between the materials.

We describe a scanning Orotron Terahertz radiation (THz) source. The operational principle is as follows: a sheet beam of electrons passes near a corrugated metal surface (Smith-Purcell grating) contained in a resonant cavity. The periodic forces on the electrons drive the cavity on its resonances in the THz regime. We describe theoretical predictions for the sheet beam parameters required and the likely performance of the device. We also describe experimental progress towards sheet-beam generation using field-emitted electrons from a carbon-nanotube array. We describe the carbon nanotube growth process and demonstrate sheet-beam current densities which exceed the predicted turn-on current density of the Orotron cavity.

We have developed a novel mm-wave spectrometer based on a Photonic Band Gap (PBG) channel-drop filter (CDF). There is a need for a compact wide-band versatile and configurable mm-wave spectrometer for applications in mm-wave communications and remote sensing. CDFs present us with a unique means for filtering frequencies at mm-waves. CDF is a novel concept allowing filtering the frequency spectra and channeling selected frequencies into separate waveguides through a PBG structure. We have designed a spectrometer with a CDF working in the frequency range of 90-130 GHz. The CDF can be connected to any type of antenna and detector. A large ground based outdoor antenna can be used for remote sensing with radars. A compact antenna can be used for indoor or space applications. The signal in the waveguide channels can be measured with any type of sensor such as a cooled bolometer or a room temperature mm-wave diode. The size of the spectrometer is under 5 inches by 5 inches and just a quarter of an inch in thick. Multiple filters can be stacked together to construct a mission specific package. We propose to construct the filter with silicon rods on a 100mm silicon wafer using MEMS technology. We will then evaluate the filter at our mm-wave laboratory to demonstrate the channeling of frequencies in a proof-of-principle experiment at 100GHz. This technology will work well for frequencies from 60GHz to 1000GHz.

Converting near infrared signals in a nonlinear medium is an attractive way to generate terahertz radiation due to the availability of near-IR lasers and nonlinear materials. However, these terahertz generation schemes are typically inefficient and are often cumbersome, which may limit their use in certain applications. We have developed and demonstrated a compact, fiber pumped optical terahertz source based difference frequency mixing (DFM) of nanosecond pulses in zinc germanium phosphide (ZGP). With this setup, we have successfully generated 2mW of average power terahertz radiation at 2.45THz. This has enabled us to perform active, real-time terahertz imaging experiments using an uncooled microbolometer array. In performing these experiments, we have also developed a theoretical model for terahertz generation based on DFM of IR pump signals. In this paper, we discuss our compact fiber pumped terahertz source technology, imaging system, model, and how we intend to overcome some of the common issues associated with optical terahertz generation.

THz optics has experienced a tremendous increase in interest among the scientific community as better THz sources and detection schemes are discovered. With recent studies in THz modulation experiment using optically excited Si opens a new possibility in constructing THz optics using an optically controlled THz SLM. Thus, various patterns, such as zone lenses, could be optically constructed and tuned in real time for used in THz beam correction. For example, optically constructed Fresnel zone lenses on high-resistivity Si can be actively tuned for focal length and chromatic aberration, which are just some possible applications of this methodology. In this paper, we will present results for an optically controlled single pixeled THz semiconductor SLM for use as a modulator and discuss extensions to applications in zone lenses, diffraction gratings, and other optical components.

This paper considers factors that affect achievable finesse for a recently demonstrated silicon-based scanning Fabry- Perot transmission filter at millimeter and sub-millimeter wavelengths. The mirrors are formed by alternating quarter-wave optical thicknesses of silicon and air in the usual Bragg configuration. Fundamental loss by lattice and free carrier absorption are considered. Technological factors such as surface roughness, bowing, and misalignment are considered for various proposed manufacturing schemes.

The development of tera-hertz sources and receivers recently has given rise to a desire to do spectroscopy on molecular rotational spectra in this region. In particular the use of semiconductor photo-mixers provides practical packaging concepts and good sensitivity for building absorption spectrometers. This paper deals with some data processing concepts associated with absorption spectra from such spectrometers. Specifically, the concepts of multivariate analysis developed and applied over the last 25 years to infrared spectroscopy appear to be very useful to the analysis of tera-hertz absorption spectra. These concepts are particularly useful to address problems associated with: spectra baseline variations, species concentration estimates and spectral cross talk assessments when multiple species are present in the measurement sample. This latter element is of particular importance in tera-hertz spectroscopy when exploring multiple species with overlapping rotation spectral bands. In this paper some basics of multivariate analysis are reviewed. The baseline signatures or backgrounds of our photo-mixer absorption spectrometer are described and processed. Algorithm results are presented that remove backgrounds and other instrument spectral artifacts. Finally, a simulated example of separating overlapping spectra is presented.

The Fresnel zone plate lens antenna, which provides advantages compared to a normal paraboloidal or spherical lens, has been extensively investigated in the millimeter-wave and terahertz regions. The advantages include reduced weight, volume, and attenuation and simplicity of design. The principal disadvantage is that the zone plate sometimes provides reduced gain compared to a true lens. Particularly at high millimeter-wave or terahertz frequencies the low loss of the zone plate more than compensates for the reduced directivity. This paper investigates the gains and far-field patterns for a number of cases and gives both the analysis and numerical results for the examples. These cases have dealt with large-angle designs, where the focal length (F) and diameter (D) are comparable (F/D = 0.3 to 2.5), unlike the typical optical examples. The antenna patterns are found to have beamwidths and first sidelobes that are similar to what one would obtain with a standard lens, given the same aperture illumination. Appropriate feed designs are also described. For best aperture efficiency the illumination taper is about 10 dB, and this gives first sidelobe levels of about -24dB for a circular aperture. Far-out average sidelobes are not as low as for a true lens, and this is where the gain is affected.