This PDF file contains the front matter associated with SPIE Proceedings Volume 8716, including the Title Page, Copyright Information, Table of Contents, and the Conference Committee listing.

The uni-traveling-carrier photodiode (UTC-PD) is a novel PD that uses only electrons as the active carriers. This feature is the key for simultaneously realizing high operation speed and high output current, which are essential for generating terahertz (THz) waves with relatively high output power. To transmit electromagnetic waves, we integrated a novel compact planar antenna with a UTC-PD and assembled it in a quasi-optical package. The fabricated module had a relatively high responsivity of 0.21 A/W and was operated in an extremely wide frequency range from 30 GHz to 1.6 THz. The detected output powers were 120 μW at 200 GHz, 17 μW at 500 GHz, and 2.9 μW at 1 THz for a photocurrent of 10 mA with a bias voltage of only -0.6 V.

This paper describes features of uncooled palm-size and real-time Terahertz (THz) imager. The THz imager and powerful THz quantum cascade laser were assembled into THz microscope with which THz images of narrow string were obtained at 4.3 and 2.0 THz. The analyses on these images show that spatial resolutions evaluated at two frequencies are consistent with Fraunhofer diffraction limit. THz imager has been applied to investigate beam patterns for a variety of THz sources. The experimental results on beam patterns show that THz imager plays an important role in developing THz sources. A method for reducing non-uniformity due to strong coherency of THz sources is finally presented.

In recent years our team has done a lot of work toward the goal of sensitive, inexpensive detectors for terahertz detection. In this paper we describe simple fabrication steps and the characterizations of uncooled Nb₅N₆ microbolometers for terahertz imaging. The best dc responsivity of the Nb₅N₆ microbolometer is –760 V/W at the bias current of 0.19 mA. A typical noise voltage as low as 10 nV/Hz^1/2 yields a low noise equivalent power (NEP) of 1.3×10^-11 W/Hz^1/2 at a modulation frequency above 4 kHz. We constructed a quasi-optical type receiver by attaching this uncooled Nb₅N₆ microbolometer to the hyperhemispherical silicon lens. Subsequently, the imaging experiment is performed using this Nb₅N₆ microbolometer receiver at a THz imaging system.

Remote sensing over long path lengths has become of greater interest in the terahertz frequency region. Applications such as pollution monitoring and detection of energetic chemicals are of particular interest. Although there has been much attention to atmospheric effects over narrow frequency windows, accurate measurements across a wide spectrum is lacking. The water vapor continuum absorption spectrum was investigated using Fourier Transform Spectroscopy. The continuum effect gives rise to an excess absorption that is unaccounted for in just a resonant line spectrum simulation. The transmission of broadband terahertz radiation from 0.300THz - 1.5THz through air with varying relative humidity levels was recorded for multiple path lengths. From these data, the absorption coefficient as a function of frequency was determined and compared with model calculations. The intensity and location of the strong absorption lines were in good agreement with spectral databases such as the 2008 HITRAN database and the JPL database. However, a noticeable continuum effect was observed particularly in the atmospheric transmission windows. A small discrepancy still remained even after accounting for continuum absorption using the best available data from the literature. This discrepancy, when projected over a one kilometer path length, typical of distances used in remote sensing, can cause a 30dB difference between calculated and observed attenuation. From the experimental and resonant line simulation spectra the air-broadening continuum parameter was calculated and compared with values available in the literature.

We report on a new method for extracting the characteristic features of covered materials, including Hexogen, in the range 0.5–1.8 THz. This time domain spectroscopy-based technique takes into account only part of the signal reflected from a covered sample, and analyzes it by Fourier transform. The obtained power spectrum has distinctive peaks that correspond to peaks measured in the transmission configuration and can be applied for further identification. We showed results obtained for the samples of hexogen, lactose, and tartaric acid covered with commonly used packaging materials such as plastic, foil, paper and cotton.

We describe preliminary design, modeling and test results for the development of a monolithic, high pixel density, THz band focal plane array (FPA) fabricated in a commercial CMOS process. Each pixel unit cell contains multiple individual THz band antennae that are coupled to independent amplifiers. The amplified signals are summed either coherently or incoherently to improve detection (SNR). The sensor is designed to operate at room temperature using passive or active illumination. In addition to the THz detector, a secondary array of Visible or SWIR context imaging pixels are interposed in the same area matrix. Multiple VIS/SWIR context pixels can be fabricated within the THz pixel unit cell. This provides simultaneous, registered context imagery and "Pan sharpening" MTF enhancement for the THz image. The compact THz imaging system maximizes the utility of a ~ 300 μm x 300 μm pixel area associated with the optical resolution spot size for a THz imaging system operating at a nominal ~ 1.0 THz spectral frequency. RF modeling is used to parameterize the antenna array design for optimal response at the THz frequencies of interest. The quarter-wave strip balanced bow-tie antennae are optimized based on the semiconductor fabrication technology thin-film characteristics and the CMOS detector input impedance. RF SPICE models enhanced for THz frequencies are used to evaluate the predicted CMOS detector performance and optimal unit cell design architecture. The models are validated through testing of existing CMOS ROICs with calibrated THz sources.

A near real-time THz-vision system is presented in the paper. The most important part of it is the THz sensors focal plane array operating at the room temperature, featuring low NEP (5pW/√Hz) and high sensitivity (1e6 V/W). Its architecture allows direct digital processing of the output signal. The system performance is upgraded with large parallel processing of up to 64 channels. The second important building block is the FM THz source used for illumination. A wide FM range, of up to ±10% of the central frequency allows using the system for various applications. The THz source is a solid-state source using a GHz range frequency synthesizer followed by frequency multipliers and microwave amplifiers. Such a compact THz source can cover the lower region of the THz spectrum, i.e. below 1THz using different frequency bands. The band selection depends on the application. Three different areas of applications are discussed in the paper: 3D imaging of hidden objects as one of the most attractive features of the presented system, an accurate range finder with the resolution within a fraction of the wave length and a narrow band CW spectrometer operating in the FM range of the source.

One of the modern problems, arising in the detection and identification of substances, is a development of criteria for the assessment of a presence of explosive (or other dangerous substance) fingerprints in investigating THz signals reflected from a sample. Obviously, criteria depend on using method for the detection and identification of the substance. Taking into account on our previous experience, we use for a solution of this problem the SDA method (method of the spectral dynamics analysis). In this case, we need, at least, developing the method for both getting requiring dynamics of spectral lines and assessment criteria and their algorithmic realization. In this paper, we show that the SDA method allows to identify the explosive under real conditions. We compare the spectral lines dynamics of THz pulse reflected from sample with the corresponding spectral lines dynamics of the THz pulse transmitted through the explosive. Used assessment and algorithm show both high probability of the substance identification and a reliability of realization in practice. Simultaneously, we discuss some problems connected with the main problem of the paper. It should be stressed that we use reference-free method for the detection of explosive. As one was emphasized in our previous papers, we do not use the main reflected pulse for the substance detection because it contains only information about the dielectric permittivity of material and does not contain the information about absorption properties of substance at using the reflection mode of THz device.

In the Dyakonov-Shur terahertz (THz) detector, nonlinearities in the plasma wave propagation in the conduction channel of a heterostructure High Electron Mobility Transistor (HEMT) lead to a constant source-to-drain voltage providing the detector output. For a small signal, the perturbation theory treatment shows that the response is proportional to the intensity of the radiation. The proportionality factor can have a resonant or a broad dependence on the signal frequency. For submicron HEMTs, the typical measured response falls within the range of 0.1 to 4.5 THz. The deviations from this relation have been studied and reported in the approximation of the local Ohm’s law and transmission line model for the non-resonant response. Here we present the results obtained with the hydrodynamic model using the electron plasma Navier-Stokes equation, thus fully accounting for the hydrodynamic non-linearity, the viscosity and pressure gradients in the detector response. The model is applicable to both resonant and broadband operations of the HEMT based plasmonic detectors. The relation between the electron channel density and gate voltage was modeled by the unified charge control model applicable both above and below the threshold voltage. The theoretical results are compared with the response measured in the short channel InGaAs HEMT and the analytical approximation. The THz source was operating at 1.63 THz and the response was measured at varying signal intensities. The response of the detector operated in the open drain mode was measured above and below the threshold. The theoretical and experimental results are in good agreement.

We report on sub-wavelength THz plasmonic split ring resonators on 2 dimensional electron gas (2DEG) at AlGaN/GaN hetero-interface and on oxide coated high mobility graphene. The investigated in this study guide THz electric field into deep sub-wavelength scale by plasmonic excitations. Propagation of a broadband pulse of EM waves was simulated by using a commercial FDTD simulation tool. The results show that split ring resonator structures can be used to guide THz into deep sub-wavelength down λ/200 and achieve relatively higher quality factors than grating gate devices by plasmonic confinement which can be used for THz detection, filtering and possibly for THz on-chip-spectrometer. Moreover, ring resonator modes supported by system can be tuned with an applied voltage to gratings.

The capability of low cost glow discharge detectors (GDDs) to detect terahertz (THz) radiation has drawn much attention recently. In order to employ them in applications such as THz imaging these studies have typically focused on the response of the GDD at specific frequencies. To better understand the spectral behavior of glow discharges, we have not only examined the response of the GDD at a specific frequency of 118 GHz, but also we examined the interaction mechanism of GDDs with THz radiation using terahertz time domain spectroscopy (THz-TDS) in a broader range of frequencies between 0.05 THz - 0.5 THz. These results show that in addition to THz induced oscillations in the plasma charge density, the structure of the GDD itself plays an important role in the detection mechanism as supported by the large response observed at a specific frequency. By increasing the bias voltage on the gap, not only is the transmission greatly reduced at this specific frequency, the results suggest that it can also be tuned. Furthermore, measurements done at 118GHz show that the GDD structure has a varying response dependent on the modulation frequency. With increasing bias voltage the reponsivity of the GDD also increases which supports previous measurements that the change in current through the plasma is due to the sub-mm wave radiation.

Interpreting the spectrum from a continuous wave frequency domain terahertz spectrometer that employs coherent detection can be challenging due to the presence of an interference pattern. We report on the continued progress of a portable, battery-operated frequency domain terahertz spectrometer with an integrated, fiber-coupled, lithium-niobate optical phase-modulator and how we achieve interference fringe elimination using phase modulation and second harmonic nulling. The implications for both transmission and reflection measurements are discussed and data on the explosive compound RDX will be presented.

Recently we introduced a Sub-THz spectroscopic system for characterizing vibrational resonance features from biological materials. This new, continuous-wave, frequency-domain spectroscopic sensor operates at room temperature between 315 and 480 GHz with spectral resolution of at least 1 GHz and utilizes the source and detector components from Virginia Diode, Inc. In this work we present experimental results and interpretation of spectroscopic signatures from bacterial cells and their biological macromolecule structural components. Transmission and absorption spectra of the bacterial protein thioredoxin, DNA and lyophilized cells of Escherichia coli (E. coli), as well as spores of Bacillus subtillis and B. atrophaeus have been characterized. Experimental results for biomolecules are compared with absorption spectra calculated using molecular dynamics simulation, and confirm the underlying physics for resonance spectroscopy based on interactions between THz radiation and vibrational modes or groups of modes of atomic motions. Such interactions result in multiple intense and narrow specific resonances in transmission/absorption spectra from nano-gram samples with spectral line widths as small as 3 GHz. The results of this study indicate diverse relaxation dynamic mechanisms relevant to sub-THz vibrational spectroscopy, including long-lasting processes. We demonstrate that high sensitivity in resolved specific absorption fingerprints provides conditions for reliable detection, identification and discrimination capability, to the level of strains of the same bacteria, and for monitoring interactions between biomaterials and reagents in near real-time. Additionally, it creates the basis for the development of new types of advanced biological sensors through integrating the developed system with a microfluidic platform for biomaterial samples.

This paper reviews recent advances in graphene active plasmonic metamaterials for new types of terahertz lasers. We theoretically discovered that when the population of Dirac Fermionic carriers in graphene are inverted by optical or electrical pumping the excitation of graphene plasmons by the THz photons results in propagating surface plasmon polaritons with giant gain in a wide THz range. Furthermore, when graphene is patterned in a micro- or nano-ribbon array by grating gate metallization, the structure acts as an active plasmonic metamaterial, providing a super-radiant plasmonic lasing with giant gain at the plasmon modes in a wide THz frequency range.

Terahertz components and devices are typically interfaced with measurement instrumentation and characterized using fixtures equipped with waveguide flanges or antennas. Such fixtures are known to introduce significant uncertainty and error in measurements. It is preferable to characterize such devices in-situ, where the device under test can be measured on-wafer, prior to dicing and separately from the circuit housing to which it is ultimately affixed. This is commonly done in the RF and millimeter-wave region with a probe station equipped with coplanar launchers. Commercial coplanar waveguide probes have generally been available to the WR-2.2 band (325—500 GHz) but few options currently exist for on-wafer measurements above these frequencies. This paper describes recent work at the University of Virginia and Dominion Microprobes, Inc. to extend on-wafer measurement capabilities to terahertz frequencies through the design and implementation of coplanar probes based on silicon micromachining. At present micromachined on-wafer probes operating to WR1.2 (600 to 900 GHz) have been demonstrated and exhibit typical insertion losses lower than 7 dB with return loss of 15 dB or greater over a full waveguide band.

A rigorous full-vectorial modal solution approach based on the finite element method is used to find the propagation properties of THz waveguides. Design approaches are presented to reduce the modal loss. Design of several THz devices, including quantum cascade lasers, plasmonic waveguides, power splitters and narrow-band filters are also presented.

A miniature neon indicator lamp, also known as a Glow Discharge Detector (GDD), costing about 50 cents, was found to be an excellent room temperature THz radiation detector. A proof of concept of 300 GHz heterodyne detection using GDD is demonstrated in this paper. Furthermore, a comparison to direct detection was carried-out and polarization effects on heterodyne detection were investigated. Preliminary results at 300 GHz showed better sensitivity by a factor of 20 with only 56 microwatt local oscillator power using heterodyne compared to direct detection. Further improvement of the detection sensitivity can be achieved if the Local Oscillator (LO) power (Plo) is increased. Effects of orthogonal polarizations of signal and local oscillator powers on heterodyne sensitivity were found to be surprisingly weak. More efficient quasi optical design for heterodyne detection is presented in this study, experimental results showed above 50% better performance compared to conventional ones.

We will present two kinds of terahertz (THz) 3D imaging performed with a continuous-wave (CW) source and phase-shifting interferometry. The first one is THz computed tomography (CT) by using phase information instead of intensity information. This minimized the effect of change in the signal strength due to diffraction and artifacts especially emerged around the edge of boundary between different materials. The second one is for the depth imaging of the surface of reflecting materials. By using a simple Michelson’s interferometer, we achieved the depth resolution of 1.1 μm, corresponding to 1/440 of the used wavelength (480 μm).

In the search for low-cost wide spectrum imagers it may become necessary to sacrifice the expense of the focal plane array and revert to a scanning methodology. In many cases the sensor may be too unwieldy to physically scan and mirrors may have adverse effects on particular frequency bands. In these cases, photonic masks can be devised to modulate the incoming light field with a code over time. This is in essence code-division multiplexing of the light field into a lower dimension channel. In this paper a simple method for modulating the light field with masks of the Archimedes’ spiral is presented and a mathematical model of the two-dimensional mask set is developed.

We report on measurements of transmission spectra of chosen materials (Hexogen, sugar, L-tartaric acid, 4-aminobenzoic acid) in the range 0.7-2.0 THz. The measurements were carried out by means of a setup, which bases on the optical parametric oscillator (OPO) combined with the hot electron bolometer (HEB). The setup consists of the commercially available tunable OPO working in the range 0.7-2.0 THz with repetition rate 53 Hz, duration of the impulse of about 20ns and energy 10nJ. The beam was detected by the fast and sensitive HEB. The spectra were compared to results obtained from a standard time domain spectroscopy (TDS) setup. Only small discrepancies between spectra measured by both methods are observed. For the range 0.7-2.0 THz typical features can be identified using both methods. Above 2 THz the TDS setup seems to have better performance in terms of signal-to-noise ratio and sensitivity.