The uni-traveling-carrier photodiode (UTC-PD) is a novel PD that uses only electrons as the active carriers. This unique feature is the key for generating continuous millimeter- and sub-millimeter waves up to the terahertz (THz) range. A compact UTC-PD module with a rectangular waveguide output port developed for operation in the F-band exhibits output powers at up to 813 GHz when used in the overmoded condition. A scaled-down rectangular waveguide output UTC-PD module for use in the J-band (220 - 325 GHz) is effective for increasing the output power at higher frequencies. A quasi-optical configuration integrating a UTC-PD and a planar antenna provides operation at higher frequencies. A device integrating a wideband log-periodic antenna exhibits a maximum output-power of 2.6 μW at 1.04 THz with good linearity. A module with a quasi-optical output port operates at frequencies of up to 1.5 THz. Devices integrating resonant narrowband dipole antennas have also been fabricated to further increase the output power. A device having a peak at 1.04 THz exhibits a maximum detected output power of 10.9 μW at 1.04 THz.

There has been enormous interest and considerable investment in recent years devoted to the development of the Terahertz (aka Submillimeter) spectral region for imaging and sensing. While a number of 'one of a kind' scientific applications (astrophysics, upper atmospheric processes, physical chemistry, molecular physics, etc.) in this spectral region have been highly successful, there are no 'public' applications of the THz. It is the central thesis of this paper that for most of the well publicized applications the lack of signature, clutter, and phenomenolgical knowledge - Signature Science - impedes progress at least as much as technical shortcomings. Thus, Signature Science represents a major scientific opportunity and a technological necessity. An important goal of Signature Science should be to provide a scientifically based body of knowledge that will make end-to-end system analyses possible at an early stage of system and application conceptualization and design. In this paper we will focus on three THz applications that have the potential to make a near term impact: 1) Imaging through obscurants, 2) Spectral signatures of gas phase chemicals, and 3) Signature quantification and analysis methods for optimal use of the information content of the spectra of solids, especially explosives.

We have studied the liquid-liquid allotropic transitions in molten sulfur using terahertz (THz) spectroscopy. Liquid sulfur is selected as an initial choice of materials because its structure and properties are well established from previous in-situ studies by one of the current investigators (and by other researchers) using a variety of physical and chemical methodologies. It is known that sulfur melts to an equilibrium mixture of octameric (S₈) rings and short chains, with a small concentration of hexameric rings (S₆). As temperature is increased, thermal energy initiates ring scission and the resulting diradically-terminated short chains undergo covalent bonding to induce polymerization at 159-166°C. Further increase in temperature causes an increase in chain length and an increase in chain species concentration until a temperature of 188°C is reached at which the long chains (~10⁶ atoms in length) undergo chain scission, and although the chains start to break up, the polymer concentration of the mixed phases still increases. We have experimentally mapped THz absorption, transmission, and reflection/scattering effects with these known transitions in liquid sulfur, as a function of temperature and wavelength.

The GaN/AlGaN multilayer structure for the active regions of terahertz quantum cascade lasers (QCLs) was grown by metal organic chemical vapor deposition (MOCVD). The surface morphology of the grown sample showed good surface quality with an average roughness of less than 1 nm. The x-ray diffraction pattern and transmission electron microscopy images showed that the well-controlled quantum cascade GaN/AlGaN layers were grown. The Fourier transform infrared spectrometer measurement showed a distinct A₁ (LO) phonon frequency at 822 cm^-1 that is red-shifted with respect to the single Al_0.2Ga_0.8N layer due to the good periodicity of the grown quantum cascade GaN/Al_0.2Ga_0.8N structure. MOCVD growth should be a viable technique for fabrication of AlGaN/GaN quantum cascade laser and phonon frequency shift should be a key indicator for the good periodicity of the grown QCL structure.

A millimeter wave source derived from a phase modulated optical signal has been developed. The spectral purity and phase control of the phase of the source allowed it to be used as a local oscillator with an astronomical millimeter wave interferometric array. The phase and amplitude stability of the correlated signals of the array are comparable to that produced by Gunn based local oscillators. The system is explained in the following article first as a simple open loop system and then as a more complex closed loop device where the phase is controlled. A mathematical description is given which predicts system behavior. The telescope correlator output graphs show phase and amplitude stability.

Recently, dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP/NMR) has emerged as a powerful technique to obtain significant enhancements in spin spectra from biological samples. For DNP in modern NMR systems, a high power continuous-wave source in the submillimeter wavelength range is necessary. Gyrotrons can deliver tens of watts of CW power at submillimeter wavelengths and are well suited for use in DNP/NMR spectrometers. To date, 140 GHz and 250 GHz gyrotrons are being employed in DNP spectrometer experiments at 200 MHz and 380 MHz at MIT. A 460 GHz gyrotron, which has operated with 8 W of CW output power, will soon be installed in a 700 MHz NMR spectrometer. High power radiation with good spectral and spatial resolution from these gyrotrons should provide NMR spectrometers with high signal enhancement through DNP. Also, these tubes operating at submillimeter wavelengths should have important applications in research in physics, chemistry, biology, materials science and medicine.

The paper discusses the feasibility of a terahertz-signal source made of AlGaN/GaN superlattice. Negative differential conductivity, electrical domain formation, current oscillations, and power efficiency of a perspective source are described. We relate the superlattice geometry and conduction band profile, distorted by polarization fields, to the oscillation frequency and power efficiency of the device. We also determine the optimal Al content, superlattice period, and the parameters of external circuit that favor sub-millimeter wave generation.

To study the reflection and transmission properties of the gratings, a novel system based on terahertz time domain spectroscopy was established. With this system, we have synchronously measured both the reflection and transmission spectra of the deep zero-order grating with subwavelength slits. In the experiments we found that the enhanced transmission spectra of the grating are complementary to the reflection spectra, which strongly depend on the geometry of the sample and the grating pose relative to the incident THz radiation, such as leaning from incident THz radiation and slits direction of the gratings out of p-polarized radiation. The experimental results agree with the theoretical analysis based on the surface plasmon polaritons and waveguide theory.

We report a simple subwavelength-diameter plastic wire, similar to an optical fiber, for guiding terahertz wave with a low attenuation constant. With a large wavelength-to-fiber-core ratio, the fractional power delivered inside the lossy core is reduced, thus lowering the effective fiber attenuation constant. In our experiment, we adopt a polyethylene fiber with a 200-500 μm diameter for guiding terahertz waves in the frequency range of 0.2-0.5 THz in which the attenuation constant is reduced to the order of or less than 0.01 cm^-1. Direct free-space coupling efficiency, as high as 20%, can be achieved by using an off-axis parabolic mirror. Furthermore, all the plastic wires are easily available in our daily life without complex processes and expensive costs.

In this paper, we demonstrate the feasibility of incorporating a modulation mechanism for negative index materials (NIM) at GHz frequency range by means of photoconductive coupling. This leads the way to a fully integrated monolithic NIM achievable by conventional microfabrication techniques. The photosensitive material was placed in the gap of the SRR structure and the response in terms of S-parameters was simulated using HFSS^TM. Only the SRR particle was simulated, without any loss of generality to the extension in concept to a NIM comprising of both negative permeability and negative permittivity.

We report our studies of the effect of microwave radiation, with a frequency much lower than that corresponding to the energy gap of the superconductor, on the performance of the NbN hot-electron bolometer (HEB) mixer incorporated into a THz heterodyne receiver. It is shown that exposing the HEB mixer to microwave radiation does not result in a significant rise of the receiver noise temperature and degradation of the mixer conversion gain so long as the level of microwave power is small compared to the local oscillator drive. Hence the injection of a small, but controlled amount of microwave radiation enables active compensation of local oscillator power and coupling fluctuations which can significantly degrade the stability of HEB mixer receivers.

Experimental results of two-dimensional homodyne terahertz interferometric imaging are presented. The performance of an N element detector array is imitated by only one detector placed at N positions. Continuous waves at 0.25-0.3 THz are used to detect concealed objects: a metal object and an RDX sample. 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.

Imaging and spectroscopy at terahertz frequencies (defined roughly as 300 GHz - 3 THz) have great potential for both healthcare and homeland security applications. Terahertz frequencies correspond to energy level transitions of important molecules in biology and astrophysics. Terahertz radiation (T-rays) can penetrate clothing and, to some extent, can also penetrate biological materials, and because of their shorter wavelengths they offer higher spatial resolution than microwaves or millimeter waves. We describe the development of a novel two-dimensional scanning, passive, terahertz imaging system based on a hot electron bolometer (HEB) detector element. HEB mixers are near quantum noise limited heterodyne detectors operating over the entire terahertz spectrum. HEB devices absorb terahertz radiation up to the visible range due to the very short momentum scattering times. The terahertz imaging system consists of a front-end heterodyne detector integrated with a state-of-the-art monolithic microwave integrated-circuit low-noise amplifier (MMIC LNA) on the same mixer block. The terahertz local oscillator (LO) signal is provided by a commercial harmonic multiplier source.

We have developed a mm wave/terahertz imaging simulation package from COTS graphic software and custom MATLAB code. In this scheme, a commercial ray-tracing package was used to simulate the emission and reflections of radiation from scenes incorporating highly realistic imagery. Accurate material properties were assigned to objects in the scenes, with values obtained from the literature, and from our own terahertz spectroscopy measurements. The images were then post-processed with custom Matlab code to include the blur introduced by the imaging system and noise levels arising from system electronics and detector noise. The Matlab code was also used to simulate the effect of fog, an important aspect for mm wave imaging systems. Several types of image scenes were evaluated, including bar targets, contrast detail targets, a person in a portal screening situation, and a sailboat on the open ocean. The images produced by this simulation are currently being used as guidance for a 94 GHz passive mm wave imaging system, but have broad applicability for frequencies extending into the terahertz region.

We present experimental studies of extraordinary terahertz (THz) transmission in subwavelength plasmonic hole arrays patterned on both metals and doped semiconductors. Transmission efficiency higher than unity was achieved when it was normalized to the area occupied by the holes. The effects of hole shape, dielectric function of metals and surrounding media, polarization dependence, and array thickness on the enhanced THz transmission in both metals and plasmonic semiconductors were demonstrated by use of THz time-domain spectroscopy. Extraordinary THz transmission was also realized in metallic arrays having thickness of only one-third of the skin depth. THz surface plasmon resonance has potential applications in THz imaging, biosensing, interconnects, and the development of next generation photoconductive antennas for THz generation and detection.

Finite element analysis, based on the vector H-field formulation and incorporating the perturbation technique, is used to calculate the complex propagation characteristics of metal-coated dielectric waveguides at THz frequencies. The propagation and attenuation characteristics of the surface plasmon modes at the metal/dielectric interfaces are presented. The effects on the modal properties of metal-clad dielectric guides with the cladding thickness and the formation of the supermodes due to the coupling between the surface plasmon modes in the presence of different surrounding materials are also investigated.

Food safety and quality concern have become more and more significant in recent years. There is therefore an increasing focus on new technologies that can be applied to food quality evaluation or safety inspection, either to simplify or speed up the checking process, or to provide additional functionality. For example, the technique of near infrared (NIR) spectroscopy has been used for the authentication of agricultural products and food samples. Terahertz (THz) radiation, or THz wave, is electromagnetic wave lies between mid-infrared and microwave radiation. During the past decade, THz waves have been used to characterize the electronic, vibrational and compositional properties of solid, liquid and gas phase materials. The main two applications in which THz fields involved are THz spectroscopy and THz imaging. Terahertz wave technology, as a new area of research, has shown its wide prospects in imaging, diagnosis, detection, and monitoring, etc. Recently, THz technology has gained a lot of attention from biological spectral analysis to bio-medical imaging due to its unique features compared with microwave and optical waves. In this paper, a brief review is given to summarize the progress of THz techqiues in the field of food inspection. The properties of THz wave, its uniqueness in sensing and imaging applications, and the prospect of this novel technology in food industry were discussed.

The kinetic energy of electrons in conductors of all types is generally neglected when computing the intrinsic impedance of structures and devices. Except at very high frequencies this is a good approximation. In superconductors the kinetic energy contributes a strong kinetic inductance contribution to the impedance. We report use of the kinetic inductance to both generate and detect ultrafast voltage transients in superconducting device structures. Superconducting micro-bridge structures are fabricated from Tl ₂Ba₂CaCu₂O₁₀ thin films which are then used to demonstrate ultrafast voltage sampling. A simple model for sampling is explained in terms of hot electron dynamics in the presence of a superconducting energy gap.

A 1.5 THz superconducting receiver has been in operation at the Receiver Lab Telescope of the Smithsonian Astrophysical Observatory in Northern Chile since December 2004. This receiver incorporates a Hot Electron Bolometer (HEB) mixer chip made from a thin film of Niobium Titanium Nitride (NbTiN), which is mounted in a precisionmachined waveguide mixer block attached to a corrugated waveguide horn assembly. With a noise temperature of around 1500 K, this receiver is sensitive enough for use in the pioneering field of ground-based terahertz spectral-line astronomy. A number of innovative techniques have been employed in the construction and deployment of this receiver. These include near-field vector beam mapping to enable accurate coupling to the telescope optics, the use of tunerless planar-diode based local oscillator unit capable of generating a few μW at 1.5 THz, and special calibration techniques required for terahertz astronomy. In this paper, we will report on the design, set-up and operation of this state-of-the-art instrument.

The ultrafast and sensitive detection of an optical beat signal generated from an optical heterodyne system in an integrated traveling-wave photoconductive detector with high-temperature superconducting (HTS) coplanar electrode lines has been introduced for generating THz signals.While the optical beat signal has been absorbed by photoconductive substrate and an HTS center strip, the THz signal is gradually bulit up on the HTS coplanar waveguide (CPW). The kinetic inductive photoresponse of the HTS strip contributes excessively in the generation of multi-THz signal while the response of the photoconductor substrate decreases with increasing THz frequency. A rigorous optical analysis of a multilayer waveguide in conjunction of optical-to-THz signal conversion analysis have been carried out to find the external conversion effciency of such a photodetector. Our simulation shows that the GaAs-based structure with YBCO electrode at1.5 μm exhibits more output THz power than its Si counterpart in 850 nm wavelength.

The criteria and conditions that give rise to the generation of optical frequency comb based on multi-signal interferences in a semiconductor optical amplifier placed in a fiber loop circuit are presented. The study includes effects of the amplifier's material parameters, electrical biasing, and gain profile in the comb frequency generation. A practical optical frequency comb generator was implemented in accordance with the theoretical model developed. The experiment produced a comb frequency spanning 4THz with a spacing of 250GHz in the 1550nm to the 1560 nm band. These experimental results were found to be in good agreement with theoretical predictions.

Materials that support strong, tunable magnetic and electric properties in the terahertz (THz) frequency range have a wide range of applications including sue in: security screening, medical imaging, bio-sensing, remote sensing, metrology, and spectroscopy. The main challenge in assembling metamaterials (MTM) aimed at higher frequency applications is the difficulty of the fabrication process. This is because metamaterials are composed of inclusions that are scaled down in size to operate at high frequencies. Consequently, a model of spilt-ring resonator(SRR)/wire MTM is proposed which can create a double-negative (DNG) passband approximately two and a half times higher than those of the conventional SRR/wire structures, by using the same dimensions. Increasing the size of the repeating structure will significantly improve the ease of fabrication when we deal with devices at high frequencies. In this paper, we demonstrate the theoretical design and experimental validations of DNG metamaterials in the THz regime. Furthermore, a novel structure is presented, which demonstrates not only DNG properties but also chirality. The form of handedness sensitive rotation of the polarization state and elliptization of visible light diffracted from the chiral structures are properties attractive to the optoelectronic technologies such as photonic bandgap crystals and microsculptured films.