There is a need 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 gamma-ray are in use. Terahertz radiation, which lies between microwave and infrared, is the last part to be exploited for want, until the last few years, of suitable sources and detectors. Terahertz imaging and spectroscopy has been shown to have the potential to use very low levels of this non-ionising radiation to detect and identify objects hidden under clothing. This paper describes recent work on the development of prototype systems using terahertz to provide new capabilities in people screening, both at security checkpoints and stand-off detection for remote detection of explosives and both metallic and non-metallic weapons.

It has been suggested that interferometric/ synthetic aperture imaging techniques, when applied to the THz regime, can provide sufficient imaging resolution and spectral content to detect concealed explosives and other weapons from a standoff distance. The interferometric imaging method is demonstrated using CW THz generation and detection. Using this hardware, the reconstruction of THz images from a point source is emphasized and compared to theoretical predictions.

Diffuse reflectance spectrum (DRS) technique is extensively used in UV/visible and middle/near infrared for characterizing/analyzing powders and samples with rough surface. In this paper, we report on the DRS investigation of explosives & related compounds in the THz region, which is more difficult because of the limitations of far infrared sources, beam splitters and detectors. We also discussed the penetration depth and the sensitivity for this technique in the THz range. THz diffuse reflectance spectra (50-680 cm^-1) were taken on a Bruker 66V/S FTIR spectrometer with a Specac diffuse reflectance accessory. A number of explosive and related compounds were investigated in both transmission and diffuse reflectance modes, and a good agreement between transmission and diffuse reflection spectra was demonstrated. Our experimental results show that DRS technique has advantages over transmission spectrum technique, such as better sensitivity and easier sample preparation. Therefore, the THz DRS has the potential for the standoff detection of explosives and related compounds in the real world applications.

Femtosecond pulse shaping for generating nearly arbitrarily shaped ultrafast optical pulses is now a well-established technology and has been widely adopted for applications ranging from high-speed communications to coherent laser control of chemical reactions. Arbitrary waveform generation (AWG) capabilities for millimeter-wave, microwave and THz electromagnetic signals, however, are quite limited. Commercial radio frequency AWG instrumentation is currently limited to ~2 GHz bandwidth. In this talk we review work at Purdue in which shaped optical pulses are used to drive an optical-to-electrical (O/E) converter. This leverages our femtosecond optical AWG technology to achieve cycle-by-cycle synthesis of arbitrary voltage waveforms in the range between a few GHz and ~1 THz. Such capabilities could open new possibilities for applications in areas such as wireless communications, electronic countermeasures, sensing, and pulsed radar. Recently our work has focused on the range from GHz to tens of GHz. A particular focus has been on the generation of signals appropriate for ultrawideband (UWB) wireless communications using "monocycle" pulses with very large fractional bandwidths. UWB technology provides high immunity to multipath interference, low probability of intercept, and high spatial resolution (for position location). Potential defense applications include tactical sensor networks and RFIF tags for inventory control. Our experiments demonstrate the ability to generate programmable monocycles with spectra that can be tailored to match emission limits and with durations and bandwidths that improve on the mainstream electronic technology. Additional potential applications include predistortion of transmit waveforms in order to precompensate distortions associated with broadband antennas and waveform optimization for enhanced target discrimination in pulsed radar.

The frequency sweep terahertz wave generator based on the resonance to the phonon polaritons in GaP has wide frequency range from 0.6THz to 6THz , high power, and high spectral purity, which enables even the fine structure measurements like those of structural defects in organic molecules. The system can be made a small size by using Cr:forsterite lasers as pump and signal sources.

There has been considerable interest in the use of the Submillimeter/THz (SMM/THz) spectral region for gas analysis and detection. This has been driven both by the importance of the application and the THz-TDS community. In this paper we will discuss and compare the attributes of an attractive alternative: cw submillimeter spectroscopy. Particular attention will be paid to sensitivity, specificity, and the investigations of harsh environments. A particularly simple system approach, the FAst Scan Submillimeter Spectroscopy Technique (FASSST), will be discussed and a compact and potentially very low cost implementation described. Results will be presented which include the analyses of complex mixtures of gases with absolute specificity.

This work demonstrates application of Fourier Transform Infrared Spectroscopy (FTIR) technique in the low terahertz frequency range of 10-25 cm^-1 to discriminate between different protein conformations and evaluate possible application of THz spectroscopy for monitoring of protein folding-unfolding process. A specific procedure developed earlier for unfolding lysozyme by salt (KSCN) precipitation and refolding the lysozyme molecules by removing of KSCN and dissolving in sodium acetate was used to prepare three different forms of lysozyme. In addition, two standard procedures were used to prepare samples in unfolded conformation: denaturation at high temperature ~95° C followed by fast freezing, and dissolution in 6 M guanidine. Thin, air dried protein films were characterized as well as material in the form of gel. Spectra reveal resonance features in transmission which represent vibrational modes in the protein samples. A great variability of spectral features for the different conformational states showed the sensitivity of vibrational frequencies to the three dimensional structure of proteins. The results obtained on liquid (gel) samples indicate that THz transmission spectroscopy can be used for monitoring folding-unfolding process in a realistic, aqueous environment.

Transmission spectra were measured over the range 90-4200 GHz for a locally sourced soil sample composed mostly of quartz sand with ~200 micron particle size. A vector network analyzer covered the spectral range 90-140 GHz. A Fourier spectrometer collected transmission spectra over the range 120 to 4200 GHz. Transmission drops to zero for wavelengths shorter than the characteristic particle size of the sample as a consequence of scattering. Transmission spectra were also measured for various liquids in the 90-140 GHz and 450-1650 GHz ranges in the interest of index matching. These liquids were mixed with the soil sample and were found to reduce scattering and increase transmission through the soil at higher frequencies. This work is relevant to mine detection using THz and millimeter wave (mmW) radiation.

Collaboration with the University of Virginia (UVa) and the University of California, Santa Barbara (UCSB) has resulted in the collection of signature data in the THz region of the spectrum for ovalbumin, Bacillus Subtilis (BG) and RNA from MS2 phage. Two independent experimental measurement systems were used to characterize the bio-simulants. Prior to our efforts, only a limited signature database existed. The goal was to evaluate a larger ensemble of biological agent simulants (BG, MS2 and ovalbumin) by measuring their THz absorption spectra. UCSB used a photomixer spectrometer and UVa a Fourier Transform spectrometer to measure absorption spectra. Each group used different sample preparation techniques and made multiple measurements to provide reliable statistics. Data processing culminated in applying proprietary algorithms to develop detection filters for each simulant. Through a covariance matrix approach, the detection filters extract signatures over regions with strong absorption and ignore regions with large signature variation (noise). The discrimination capability of these filters was also tested. The probability of detection and false alarm for each simulant was analyzed by each simulant specific filter. We analyzed a limited set of Bacillus thuringiensis (BT) data (a near neighbor to BG) and were capable of discriminating between BT and BG. The signal processing and filter construction demonstrates signature specificity and filter discrimination capabilities.

The detection of buried anti-personnel mines has presented problems for current mine detection methods, such as ground penetrating radar. Terahertz (THz) radiation provides the ability to obtain higher resolution images as well as the ability to obtain spectroscopic information on the explosives in and around the buried mines. Propagation of THz in granular media is studied using techniques adapted from material science for foams and ceramics and adapted to sand and soil. We then give early results of 2-D reflection imaging of objects buried in sand. A terahertz time domain reflection system with a GaAs photoconductive emitter, a ZnTe electro-optic detector, and a rapid delay scanning mechanism was used to produce and collect the transmitted THz signal.

We model a new THz laser device structure based on a semiconductor quantum dot (QD) gain medium, where the lasing occurs through discrete conduction states. An ensemble of QDs is selectively placed in a high quality cavity, called a microdisk, which is resonant with an intersublevel QD transition. We simulate the rate equations governing lasing and discuss a variety of processes affecting lasing including nonradiative recombination and the ground state decay rate.

We designed the Bragg reflectors and 2-D photonic crystals using the R-Soft BandSOLVE software. Following our initial designs, we fabricated the Bragg reflectors and 2-D photonic crystals on Si wafers by using two techniques, i.e. drilling holes using a UV laser and deep reactive ion etching. We then characterized the fabricated structures by measuring both the reflection and transmission spectra using the widely-tunable monochromatic THz source developed by us as well as FTIR. We analyzed and compared our results based on these two different techniques. We also investigated the dependence of the bandgaps on the fluctuation of the diameters of the air holes using the R-Soft software.

Grating gated field effect transistors (FETs) are potentially important as electronically tunable terahertz detectors with spectral bandwidths of the order of 50 GHz. Their utility depends on being able to 1) use the intrinsic high speed in a heterodyne mixer or 2) sacrifice speed for sufficient sensitivity to be an effective incoherent detector. In its present form the grating gated FET will support IF frequencies up to ~10 GHz, an acceptable bandwidth for most heterodyne applications. By separating the resonant plasmon absorption from the responsivity mechanism, it appears that a tuned, narrow terahertz spectral band bolometer can be fabricated with NEP ~ 10^-11 watts/√Hz and response times of the order of 30 msecs, useful in a passive multispectral terahertz imaging system.

Doping of the lead telluride and related alloys with the group III impurities results in appearance of the unique physical features of a material, such as persistent photoresponse, enhanced responsive quantum efficiency (up to 100 photoelectrons/incident photon), radiation hardness and many others. We present the physical principles of operation of the photodetecting devices based on the group III-doped IV-VI including the possibilities of a fast quenching of the persistent photoresponse, construction of the focal-plane array, new readout technique, and others. The advantages of infrared photodetecting systems based on the group III-doped IV-VI in comparison with the modern photodetectors are summarized. The spectra of the persistent photoresponse have not been measured so far because of the difficulties with screening the background radiation. We report on the observation of strong persistent photoconductivity in Pb_0.75Sn_0.25Te(In) under the action of monochromatic submillimeter radiation at wavelengths of 176 and 241 microns. The sample temperature was 4.2 K, the background radiation was completely screened out. The sample was initially in the semiinsulating state providing dark resistance of more than 100 GOhm. The responsivity of the photodetector is by several orders of magnitude higher than in the state of the art Ge(Ga). The red cut-off wavelength exceeds the upper limit of 220 microns observed so far for the quantum photodetectors in the uniaxially stressed Ge(Ga). It is possible that the photoconductivity spectrum of Pb_1-xSn_xTe(In)covers all the submillimeter wavelength range.

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. Time domain terahertz imaging can be employed in reflection mode to image with sub millimeter resolution. Previously, single pixel acquisition times for THz waveforms was typically 20 Hz with time records of approx 80 picoseconds, which typically restricted imaging time to hours for areas on the order of 1 square foot, limiting the field practicality of the equipment. We describe and demonstrate advanced imagers with 100 Hz --> 320 picosecond, and 4000 Hz -- 20 picosecond waveform records. These systems have been demonstrated to image >600 pixels/second from a single channel. Such a system, combined with a 32 channel linear THz array, could image a 1 square foot area with 1 mm resolution in <5 seconds, performing a shoe explosives detection image in a short period of time.

The design, construction, and investigation of a compact reflectometer suitable for measuring return loss magnitude and phase of biological materials and chemical agents at submillimeter wavelengths is presented. The instrument, which consists of a section of rectangular waveguide and an ensemble of Schottky diode power detectors is designed as a proof-of-principle demonstration and is a relatively simple implementation of the six-port analyzer originally investigated by Engen and coworkers. Design considerations for the reflectometer are presented and measurements in the 270 GHz to 320 GHz range are discussed. In addition, preliminary return loss measurements on sample biological materials (salmon and herring DNA) in the millimeter-wave region are presented and described.

We have achieved the first demonstration of a low-noise heterodyne array operating at a frequency above 1 THz (1.6 THz). The prototype array has three elements, consisting of NbN hot electron bolometer (HEB) detectors on silicon substrates. We use a quasi-optical design to couple the signal and local oscillator (LO) power to the detector. We also demonstrate, for the first time, how the HEB detectors can be intimately integrated in the same block with monolithic microwave integrated circuit (MMIC) IF amplifiers. Such focal plane arrays can be increased in size to a few hundred elements using the next generation fabrication architecture for compact and easy assembly. Future HEB-based focal plane arrays will make low-noise heterodyne imaging systems with high angular resolution possible from 500 GHz to several terahertz. Large low-noise HEB arrays are well suited for real-time video imaging at any frequency over the entire terahertz spectrum. This is made possible by virtue of the extremely low local oscillator power requirements of the HEB detectors (a few hundred nanowatts to a microwatt per pixel). The operating temperature is 4 to 6 K, which can be provided by a compact and mobile cryocooler system, developed as a spin-off from the space program. The terahertz HEB imager consists of a computer-controlled optical system mounted on an elevation and azimuth scanning translator which provides a two-dimensional image of the target. We present preliminary measured data at the symposium for a terahertz security system of this type.

Calculated terahertz gain for periodically delta-doped p-Ge films with vertical and in-plane transport and an orthogonal magnetic field are compared. Gain as a function of structure period, doping concentration, field strength, and temperature is calculated using distributions determined from Monte Carlo simulations. Both transport schemes achieve spatial separation of light holes from impurity layers and the majority of heavy holes, which significantly increases light hole lifetime and gain compared with bulk p-Ge lasers. For in-plane transport, an optimum doping period of 1-2 μm and a 10-fold increase in gain over bulk p-Ge are found. For vertical transport, the optimum period is 300-400 nm, and the gain increase found of 3-5 times bulk values is more modest. However, it is found that gain can persist to higher temperatures (up to 77 K) for vertical transport, while the in-plane transport scheme appears limited to 30-40 K.

The phase-correcting Fresnel zone plate provides lens-like focusing and imaging of electromagnetic waves by means of diffraction instead of refraction, often referred to as diffraction optics. The zone plate has seen extensive investigation and use at microwave and millimeter-wave frequencies, and recently has been applied in the terahertz region. These cases have dealt principally 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 planar zone plate, in particular, offers the advantages of simplicity of design and construction, low loss, low weight, and low cost, while giving performance similar to that of a refractive lens. As one goes to terahertz frequencies, ease of construction becomes more difficult. The attenuation in conventional low-loss materials increases at higher frequencies, and dimensional tolerances become smaller, making fabrication more difficult. Although earlier designs employing polystyrene have been built and tested at frequencies up to 280 GHz, higher frequency designs are simpler to fabricate and have lower loss if low dielectric constant materials are used. This investigation addressed designs for terahertz frequencies. The optimization of the zone plate has also been examined, and improvement has been found for radial compression, where the radii of the zone boundaries are slightly shortened.

A biological(bio)-molecular inspired electronic architecture is presented that offers the potential for defining nanoscale sensor platforms with enhanced capabilities for sensing terahertz (THz) frequency bio-signatures. This architecture makes strategic use of integrated biological elements to enable communication and high-level function within densely-packed nanoelectronic systems. In particular, this architecture introduces a new paradigm for establishing hybrid Electro-THz-Optical (ETO) communication channels where the THz-frequency spectral characteristics that are uniquely associated with the embedded bio-molecules are utilized directly. Since the functionality of this architecture is built upon the spectral characteristics of bio-molecules, this immediately allows for defining new methods for enhanced sensing of THz bio-signatures. First, this integrated sensor concept greatly facilitates the collection of THz bio-signatures associated with embedded bio-molecules via interactions with the time-dependent signals propagating through the nanoelectronic circuit. Second, it leads to a new Multi-State Spectral Sensing (MS3) approach where bio-signature information can be collected from multiple metastable state conformations. This paper will also introduce a new class of prototype devices that utilize THz-sensitive bio-molecules to achieve molecular-level sensing and functionality. Here, new simulation results are presented for a class of bio-molecular components that exhibit the prescribed type of ETO characteristics required for realizing integrated sensor platforms. Most noteworthy, this research derives THz spectral bio-signatures for organic molecules that are amenable to photo-induced metastable-state conformations and establishes an initial scientific foundation and design blueprint for an enhanced THz bio-signature sensing capability.

We present a theoretical study of two-terminal nitride-based oscillators utilizing two different hot-electron transport regimes determined by the interaction of the electrons with polar optical phonons, which are capable to generate current/voltage oscillations in the THz-frequency range. The first is the limited space-charge accumulation (LSA) regime based on the negative differential resistance (NDR) in a bulk-like GaN structure at room temperature at high electric field E > Et (Et ≈ 150 kV/cm). The second is the streaming regime in a quantum well (QW) structure in moderate electric field (1-10 kV/cm) at the nitrogen temperature or higher. The latter corresponds to the optical-phonon transit-time (OPTT) resonance and typically does not lead to an NDR at zero frequency. We show that for both regimes, real part of the electron dynamic mobility can be negative within certain THz-frequency windows, whose location and width depend on E. For a 100-nm n-GaN diode with a cross-section of 500 μm² and the electron density of 1×10¹⁷ cm^-3, the generated microwave power is estimated to be ≈ 0.6 W with the dc-to-rf conversion efficiency ≈ 9 % and the magnitude of the NDR of -1.3 Ω. When the streaming transport is realized in the QW channel, the generated power is estimated to be about 350 mW with the efficiency of few percent for a ten QWs GaN-based structure. Hence, the investigated transport mechanisms provide efficient mean to achieve very high-frequency microwave generation in the nitrides.

Several problems, almost impossible to defeat, among them, heat removal, addressing small devices, and fuzziness at atomistic scales confronts standard CMOS electronics. What we are concluding after intensive research is that the material is not the major problem but the way how we encode information is what can allow us to continue the steady exponential grow in computational power know as the Moore's law. Although molecules and nanoclusters are the alternative for scaling down devices, we need to develop scenarios for information coding and transfer in molecular circuits able to operate at integration densities and speeds orders of magnitude higher than in present integrated circuits. A simple scaling down using the same scenarios being used in today's microelectronics devices does not offer much hope at the atomistic and nanoscopic levels. We proposed two scenarios: One is based on the characteristic vibrational behavior of molecules and clusters and the other is based on their molecular electrostatic potentials. It is proposed that these two scenarios can be used for molecular signal processing and transfer in molecular circuits; theoretical demonstrations using computational techniques are presented for these two paradigms. The molecular electrostatic potential in the neighborhood of a molecule has very well defined zones of positive and negative potential that can be manipulated to encode information and vibrational modes of long molecules can allow us to transfer signals. Both scenarios allow very lower energies, higher speeds, and higher integration densities than in any other technology. A review of our search for other scenarios for coding, processing and transport of information is provided.

The THz is unique among spectral regions because of the relative infancy of its commercial applications. Much of this infancy has been due to the well known difficulties of generating and detecting radiation. However, the enormous number of important applications in each of the other spectral regions has resulted at least as much from their large in-vestment in systems and applications development - an 'X' factor - as from the technological maturity of the spectral region. Examples in the radio region include magnetic resonance imaging (rf + 'X' = shaped magnetic fields, rf pulse sequences, and signal processing) and cruise missiles (rf = 'X' = rocket and guidance system). In the visible, Night Vision (light = 'X' = electron multiplication and fluorescence) serves as an example. To grow to maturity, the THz needs not only to optimize its technology for native applications (imaging through ob-scuration, chemical sensing, etc.), but to integrate its attributes with other technologies to address a broader range of challenges. In this paper we will discuss the underlying physics of interactions in the THz to see how they lead to both the attractive and limiting features of the spectral region, while at the same time providing hints about how to overcome these limitations by considering 'X'. Specific examples of 'X' will be provided and the authors will welcome comments, suggestions, and ideas from the audience.

We present the development of a set of electron and hole quantum transport equations for barrier devices with dilute magnetic semiconductor (DMS) regions. The equations are developed from the time dependent equation of motion of the density matrix equation in the coordinate representation, leading to both the transient spin Wigner equations and the 'classical' spin drift and diffusion equations for high 'g' factor DMS materials. The role of DMS layers is illustrated for two structures; one where the DMS layer is confined to the first barrier, and another with DMS emitter and collector barriers. In each case we obtain the spinup and spindown carrier and current distributions, from self-consistent solutions to the transient spin dependent Wigner equation. Negative differential conductance as well as the significant unequal spinup and spin down charge distributions are obtained.

The development of nanowire and nanotube FETs for high frequency applications faces a challenge of impedance matching, due to the inherent mismatch between the resistance quantum (≈ 25 kΩ) typical of nanodevices, and the characteristic impedance of free space (≈ 377 Ω) typical of RF circuits. One possible solution is to use parallel nanotube or nanowire FETs to decrease the input impedance, and increase the drive current. In this paper, we present our progress towards this goal using aligned arrays of nanotube FETs. Initial studies on randomly oriented CVD grown devices give mobilities of 4 cm²/V-s. These initial devices carry ≈ 0.25 mA of current. Even higher mobilities (hence very high operational frequencies up to THz) should be possible with aligned nanotube FETs.

By coupling a radio-frequency single-electron transistor (RF-SET) to a quantum dot (QD) in a GaAs/AlGaAs heterostructure, we have succeeded in detecting the tunneling of individual electrons on and off the QD on time scales as short as one microsecond. Using charge detection to probe the state of the QD allows us to nearly isolate the dot from its leads, thereby minimizing decoherence-inducing effects of the environment. We have extended these charge detection techniques to double quantum dots (DQDs) that can simultaneously be used to characterize the backaction of the RF-SET. The combined RF-SET/DQD system is well-suited to the development of charge- or spin-based quantum bits, and to investigation of the quantum measurement problem.

Optically controlled modulation of broadband THz radiation with a comparably uniform spatial distribution is demonstrated in a Si-based semiconductor structure with moderate doping. Using THz Time-Domain Spectroscopy a maximum intensity modulation of more than 99% was demonstrated for a spectrum ranging from 50GHz to 3.5THz with 3dB attenuation already for optical excitation as low as only 5mW. The uniformity of the modulation was measured and compared to the THz beam profile.

Through the support of the US Army Research Office we are developing terahertz sources and detectors suitable for use in the spectroscopy of chemical and biological materials as well as for use in imaging systems to detect concealed weapons. Our technology relies on nonlinear diodes to translate the functionality achieved at microwave frequencies to the terahertz band. Basic building blocks that have been developed for this application include low-noise mixers, frequency multipliers, sideband generators and direct detectors. These components rely on planar Schottky diodes and integrated diode circuits and are therefore easy to assemble and robust. They require no mechanical tuners to achieve high efficiency and broad bandwidth. This paper will review the range of performance that has been achieved with these terahertz components and briefly discuss preliminary results achieved with a spectroscopy system and the development of sources for imaging systems.

Terahertz spectroscopy can be used to detect explosives and related compounds by the unique absorption lines in the far infrared region. We will present results in both transmission and reflection geometries using a conventional time-domain terahertz spectroscopy system. We illustrate the importance of a uniform reference, and uniform sample preparation. Some problems can be avoided by generalizing this technique to detection with narrow linewidth terahertz illumination and a square law (intensity) detector.

The traditional implementation of resonant tunneling diodes (RTD) as a high-frequency power source always requires the utilization of negative-differential resistance (NDR). However, there are inherent problems associated with effectively utilizing the two-terminal NDR gain to achieve significant levels of output power. This paper will present a new design methodology where resonant tunneling structures (RTS) are engineered to exhibit electronic instabilities within the positive-differential-resistance (PDR) region. As will be demonstrated, this approach utilizes a microscopic instability that alleviates the need to reduce device area (and therefore output power) in an effort to achieve low-frequency stabilization.