The spatial, temporal, and spectroscopic characteristics associated with pulsed THz (100 GHz - 70 THz) radiation provide this emerging technology with the potential for reliable identification of buried objects such as non-metallic landmines. With a suitable integration of these attributes, one can envision a THz detection platform that provides: (1) accurate identification of buried objects, and (2) a source-to-sample working distance that is sufficient for remote sensing applications. In our preliminary laboratory studies, we have demonstrated the detection capabilities of THz radiation by imaging a small rubber object embedded in a moist, sand-like soil. Despite the significant attenuation of the THz radiation via water absorption and particle scattering, the initial transmission results showed that pulsed THz imaging could identify the non-metallic object when buried in a few inches of soil. The sub-millimeter resolution observed in our THz images illustrates the potential to discriminate landmines from other buried objects. Finally, THz calculations and measurements determined that our current THz source and detector has sufficient SNR to detect a buried object to a depth of 6 inches in moist sand.

Pulsed THz imaging systems have a number of potential advantages in inspection applications. They provide amplitude and phase information across a broad spectral range in the far-infrared, and many common packaging materials are relatively transparent in this frequency range. We use T-ray imaging to allow the identification of different powdered materials concealed inside envelopes. Using the terahertz spectral information we show that different powders may be uniquely identified. Different thicknesses of the powders are imaged to investigate the influence of scattering on the measured THz pulses and the classification model is extended to allow it to identify different materials independent of the material thickness.

Terahertz (THz) time-domain spectroscopy (TDS) is a powerful measurement tool for characterizing materials with potential fingerprint capability. Due to its pulsed nature, the spectral resolution of THz-TDS is limited by its temporal scanning measurement and its dynamic range. A novel THz-TDS system with a large signal-to-noise ratio (SNR) improves the spectral resolution. Techniques that will enhance the performance of THz-TDS are demonstrated.

Water, at both the liquid and gas phase, maintains a high absorption coefficient in the terahertz (THz) frequency range. As a result, a major limitation of THz time-domain spectroscopy (THz-TDS) for real-world applications is water attenuation. The humidity in the atmosphere affects THz waves (T-ray) for long distance measurement and tracing materials, such as explosive materials. We measure air at various humidity and we report how humidity affects THz-TDS measurement. We also report the changes to spectrum amplitudes by measuring water vapor absorption in a vacuum chamber.

The growing and immediate threat of biological and chemical weapons has placed urgency on the development of chemical and biological warfare agent (CWA/BWA) screening devices. Specifically, the ability to detect CWA/BWA prior to deployment is paramount to mitigating the threat without exposing individuals to its effects. SPARTA, Inc. and NIST are currently investigating the feasibility of using far-infrared radiation, or terahertz (THz, 1 THz = 10¹² Hz) radiation, to non-invasively detect biological and chemical agents, explosives and drugs/narcotics inside sealed containers. Small-to-medium sized molecules (3-100 atoms) in gas, liquid and solid phases consistently exhibit identifiable spectral features in the far-IR portion of the spectrum. Many compounds associated with weapons of mass destruction are made up of molecules of this size. The THz portion of the spectrum lies between visible light and radio waves, allowing for partial transmission of 0.3-10.0 THz (30-1000 μm, 10-330 cm^-1) light through most common materials. Therefore, transmission measurements of THz light can potentially be used to non-invasively detect the presence of CWA/BWA, explosives and drugs in the pathway of a THz radiation beam.

Recent events have accelerated the quest for ever more effective security screening to detect an increasing variety of threats. Many techniques employing different parts of the electromagnetic spectrum from radio up to X- and gammaray are in use. Terahertz radiation, which lies between microwave and infrared, is the last part to be exploited for want, until recently, of suitable sources and detectors. This paper describes practical techniques for Terahertz imaging and spectroscopy which are now being applied to a variety of applications. We describe a number of proof-of-principle experiments which show that Terahertz imaging has the ability to use very low levels of this non-ionising radiation to detect hidden objects in clothing and common packing materials and envelopes. Moreover, certain hidden substances such as plastic explosives and other chemical and biological agents may be detected from their characteristic Terahertz spectra. The results of these experiments, coupled with availability of practical Terahertz systems which operate outside the laboratory environment, demonstrate the potential for Terahertz technology in security screening and counterterrorism.

We have conducted visible pump-THz probe experiments on single wall carbon nanotubes (SWCNTs) on quartz substrates. Our results suggest an upper limit to the carrier-lifetime, which is on the order 1.5ps, limited only by the THz pulse duration. These experiments were repeated for ion-implanted, 3-4nm Si nanoclusters in quartz for which the carrier lifetime was also assessed at 1.5ps. THz time-domain spectroscopy (THz-TDS) of SWCNTs revealed that the THz pulse peak transmission changed under optical illumination.

A proposed, non-invasive, means to detect and characterize concealed biological and explosive agents in near real-time with a wide field-of-view uses spatial imaging of their characteristic transmission or reflectivity wavelength spectrum in the Terahertz (THz) electro-magnetic range (0.1-3 THz). Neural network analyses of the THz spectra and images will provide the specificity of agent detection and reduce the frequency of false alarms. Artificial neural networks are mathematical devices for modeling complex, non-linear functionalities. The key to a successful neural network is adequate training with known input-output data. Important challenges in the research include identification of the preferred network structure (e.g. multi-layer perceptron), number of hidden nodes, training algorithm (e.g. back propagation), and determination of what type of THz spectral image pre-processing is needed prior to application of the network. Detector array images containing both spectral and spatial information are analyzed with the aid of the Neurosolutions(TM) commercial neural network software package.

Calabazas Creek Research, Inc. is funded by the National Aeronautics and Space Administration to develop advanced backward wave oscillators that incorporate energy recovery, air cooling and improved performance. An improved coupler transforms the generated RF power to a high-purity, Gaussian output mode. The construction of a 600-700 GHz BWO is currently underway with higher frequency sources in development. Simulations predict 6-8 mW of RF power over a 100 GHz bandwidth.

This paper describes a concept to generate coherent THz radiation in a semiconductor diode device using phonon generation via high-mobility electrons in semiconductor quantum well heterostructures. The theoretical basis for pumping both acoustic and optical phonons by high-mobility, two-dimensionally confined electrons has been established over the past decade. The electrons drift parallel to the quantum well heterojunction, and because their drift velocity exceeds either the local velocity of sound or the phase velocity of optical phonons in the crystal, energy is transferred from the electrons to the phonons (Cherenkov radiation). Strong confinement of both electrons and optical phonons in the quantum well leads to highly efficient energy transfer from high mobility electrons to coherent phonon waves. Plasmon oscillations created by coherent phonons in a polar material (such as GaAs or InP) create propagating THz electromagnetic fields. This process is analogous to the physical process that is the basis of a laser: multi-level pumping, stimulated emission, and a selection of one mode at the expense of the other modes. This paper describes a design approach to design structures that will produce required electron velocities and bias fields, for phonon generation through electrical pumping. This paper will also discuss the applicability of incorporating acoustic mirrors for a high finesse phonon cavity, and approaches for outcoupling the THz radiation.

In this Paper we investigate a tunable metallic photonic crystal filter with a novel mechanical tuning method, suitable for use in terahertz frequency applications. Tuning has been demonstrated in a micrometer-driven prototype at 70 - 110 GHz in accordance with rigorous full-vector electromagnetic simulations (finite-difference time-domain). The measured pass band has a Q of 11 and can be tuned over a 3.5 GHz range. The insertion loss is only 1.1 to 1.7 dB, while the stop band attenuation is >10 dB. The filter has the advantages of inexpensive, robust and compact construction and tunable operation that readily scales to any desired terahertz frequency.

At terahertz frequencies the attenuation of conventional lenses can be very high. In addition, the fabrication tolerances for standard curved lenses (e.g., hyperboloid in shape) make construction difficult and costly. In contrast, zone plate lenses are thin (typically on the order of a wavelength) and therefore have low loss. In addition, zone plates are planar, and the flat surfaces are easy to fabricate. They are also much lower in weight than a true lens. These considerations have led to the development of several designs at frequencies to 600 GHz. All typical lens materials show an increase in loss tangent at frequencies above 100 GHz, and for some the loss increases at an undesirable rate. For example, polystyrene has much greater loss than Teflon at 600 GHz. Thus, material choices are a consideration, and dielectric constant is also a part of this consideration. At 300 GHz the wavelength is 1 millimeter. A zone plate with this thickness poses support problems, but by the use of material with a dielectric constant of 1.05, the structure becomes about 10 times thicker and therefore more structurally feasible. These options are described, and design cases will be given. Various examples have already been designed, built, and tested at frequencies near 100, 140, 210, 235 and 280 GHz, and their characteristics will be summarized. The result is a compilation of recommendations for usable designs, with information on losses, beamwidths, feed arrangements, and construction considerations, as well as comparisons to conventional lenses.

Terahertz imaging has the potential to reveal concealed explosives; metallic and non-metallic weapons (such as ceramic, plastic or composite guns and knives); flammables; biological agents; chemical weapons and other threats hidden in packages or on personnel. Because terahertz imaging employs safe non-ionizing radiation that penetrates clothing, people may be routinely scanned as well as packages. Images can have sub-millimeter resolution, superior to longer wavelength techniques. Explosives, chemical weapons, and biological agents may posses a spectral fingerprint in the terahertz regime. The application of commercially available time domain terahertz spectroscopy equipment to imaging of concealed threats within packages is shown to penetrate common packing materials and clearly image common plastic and metal threat objects.