Using a reflection-based pulsed THz imaging system built upon our ErAs:GaAs photoconductive switch and a gated receiver, we quantify image quality at different detection bands (centered at 100, 400, and 600 GHz). Zero-bias Schottky diode detectors mounted in various waveguide sizes are used to tune the operational frequency bands of the imaging system, while the rest of the imaging system remains unchanged. The image quality is quantified by applying an optical character recognition (OCR) algorithm on THz images of 8-by-10 mm copper letters on a fiberglass substrate. Using the OCR success rate as a metric, we see a fivefold improvement in image quality from a 400 GHz to a 600 GHz imaging system, while our 100 GHz images do not produce any correct OCR results. In a comparison experiment performed at 600 GHz, the image signal-to-noise ratio (SNR) is degraded by placing increasing numbers of denim sheets (5.4 dB decrease in signal per layer) into the beam path. We find that the OCR success rate is roughly constant from one sheet to four sheets of denim (33-25 dB SNR) and then drops off sharply starting at five denim sheets.

Amorphous Nb-Si has been previously demonstrated as a Josephson junction barrier material for Nb-based superconducting voltage standard circuits, including both DC programmable and AC Josephson voltage standards operating at frequencies up to 20 GHz. This material was chosen so that the junctions could be fabricated in vertical stacks, increasing the number of junctions in an array, which in turn increases the output voltage of the circuits. This barrier material may also be used to create higher-speed junctions, because the same factors that lead to improved stacks also lead to more reproducible junctions with thin, insulating barriers. Recently, a collaboration between the Physikalisch-Technische Bundesanstalt (PTB) and the National Institute of Standards and Technology (NIST) produced 1 V and 10 V programmable Josephson voltage standard chips operating at 75 GHz that use these junctions. In this paper, we demonstrate junctions with characteristic frequencies approaching 500 GHz and observed Josephson phase locking at frequencies of 400 GHz and 800 GHz. These junctions are promising for applications in high-speed superconducting digital electronics.

Over the past several years, many groups have developed both millimeter-wave as well as terahertz imaging systems for concealed weapons detection. Typically, systems operating at the millimetre-wave range benefit from good transmission of these frequencies through common clothing materials, but provide only modest spatial resolving power at distances larger than a few meters for practical aperture sizes (dap < 1 m). Hence, these existing systems fall in the category of anomaly detectors, i.e. they intrinsically lack the performance to discriminate threat items from innocuous objects, such as cell phones, mp3 players and the like. Moreover, the radiometric performance of a passive imager has to be better than 0.5 K per frame for sufficient signal-to-noise ratio. In this joint Euro-American effort, we are developing a passive ~0.3 THz - 1 THz camera demonstrator capable of sub-Kelvin thermal resolution at video frame rates. The cryogen-free system utilizes a linear array of cryogenic antenna-coupled vacuum-bridge microbolometers, coupled to innovative all-reflective conical scanning optics and room temperature read-out electronics. First imaging results from the video rate system will be presented.

I briefly describe some recent THz cameras we have developed, emphasizing the most recent: a conically-scanned, 128-element linear array of ultrawideband, 4K microbolometers.

Conventional material measurements of transmission and reflection in the millimeter-wave and terahertz frequency range do not differentiate between scattering and absorption, grouping effects from both mechanisms together into "loss". Accurate knowledge of the balance between scattering and absorption is critical in applications such as radiometric scene modeling for concealed object detection, where evaluation of object detectability depends strongly on the amount of scattering due to concealers such as clothing. We describe an experimental setup for the measurement of spatial bidirectional reflectance distribution function (BRDF). Previous measurements have shown extremely low-level grating lobes from periodic clothing materials such as corduroy, around 30 dB below the transmitted beam. To adequately address this issue of high dynamic range, we utilize a cryogenic antenna-coupled microbolometer for detection. We present data on several types of expanded polystyrene, a common structural material for systems and experiments in this frequency range. In these measurements of BRDF, transmission agrees with previous measurements, and the balance between low and high angle scattering, specular reflectance, and absorption is examined.

Passive imaging of concealed objects at stand-off distances in excess of a few meters requires both excellent spatial, thermal and temporal resolution from the terahertz imaging system. The combination of these requirements while keeping the overall system cost at a reasonable level has been the motivation for this joint work. The THz imaging system under development is capable of sub-Kelvin NETD at video frame rates. In this paper we report the first imaging results from a 16-pixel array of superconducting antenna-coupled NbN vacuum-bridge microbolometers, operated within a cryogen-free, turn-key refrigerator. In addition to the system overview, we shall also address the uniformity of the detectors and present passive indoors raster-scanned imagery.

We present ultrawideband imagery obtained with modular, 8-element, superconducting Nb microbolometer arrays. Conically scanned images are presented and compared with raster-scanned images obtained on the same arrays and from similar NbN arrays at VTT. Statistical data on detector non-uniformity, and methods for mitigating and compensating it are described. Low-noise readout is accomplished with room-temperature electronics using the transimpedance scheme of Pentilla et al. Characterization of spatial resolution, noise-equivalent temperature difference, and spectral response is done using metrology tools - standard targets, mm-wave blackbodies, and variable filters - that have been developed at NIST for this purpose.

In gas spectroscopy, chemicals can be identified by the set of frequencies at which their absorption lines occur. The concentration can be quantitatively estimated from the intensity of any of the absorption lines. The sensitivity of the spectrometer, i.e., the minimum detectable concentration, is ideally limited by the ratio of the source power to detector noise-equivalent power. In practice, the sensitivity is usually orders of magnitude worse due to systematic effects. In this work we built a simple gas terahertz transmission spectrometer to analyze how the source output power stability, the detector sensitivity, and atmospheric pressure affect its sensitivity. As a test gas we used methyl chloride in a mixture with air and modifid the widths of the absorption lines by changing partial pressure of air. This demonstration of a simple absorption spectrometer gives us insight into the approach to making a highly sensitive terahertz spectrometer.

Operating in reflection and using single pixel, whiskbroom scanning to generate images, we report on a THz imaging system with a spatial resolution of 1 mm and a post detection SNR of 67 dB. The source in the system is an ErAs:GaAs photoconductive switch, , and the detector is a waveguide mounted, zero-bias Schottky diode operating at a center frequency of 600 GHz and a 3 dB bandwidth of 200 GHz. The high output power of the photoconductive switch and large instantaneous bandwidth of both switch and detector allow the system to achieve this high post detection SNR in a 60 Hz bandwidth when reflecting off a specular metal target at a standoff of 4 cm. The spatial resolution is superior to that predicted by the standard Rayleigh criterion as expected from the systems' Gaussian (non uniform) illumination, but is inferior to the Sparrow Criterion due to the presence of system noise, and discrete rounding of sample amplitudes.

The narrow terahertz (THz) features in crystalline biotin and lactose monohydrate observed in recent experimental studies are considered by solid-state density functional theory (DFT) calculations. The lowest-frequency THz features in both solid-state biotin and lactose monohydrate are assigned to external hindered rotational modes and not to the lowest-frequency internal modes predicted from isolated-molecule calculations. The motions of the molecules associated with these narrow THz features and the interactions between molecules in the hydrogen-bonded networks of these molecular crystals are discussed, and comparisons are made to similar studies on molecular crystals not exhibiting strong intermolecular interactions.

The authors report on the room-temperature characterization of a narrow absorption signature in an organic molecular solid (alpha-lactose monohydrate) centered around 530 GHz and having a full width at half maximum as small as 23 GHz. Three different spectrometric techniques were applied, two of them (tunable photomixing and tunable frequency multiplication) having a high enough resolution (< 100 MHz) to accurately curve fit the signature and determine that it is much better described by a Lorentzian than a Gaussian function. The Lorentzian model provides an estimate of 14 ps for the damping time associated with this signature.

We investigate the spectral response of a THz imaging system based on ultrawideband cryogenic microbolometers. The bandwidth if this system, nominally 0.2 - 1.8 THz, is broad enough to span large variations (>10 dB) in clothing transmittance and diffraction-limited spatial resolution (factor of x8), factors that are presumably partly responsible for the unusually high quality of the images taken with it. The chief tools we have used for this are a simple THz monochromator based on a specially designed frequency selective surface, and a specially designed blackbody source that provides an accurately known power spectral density over the full bandwidth of the imager. Two completely independent measurements of the microbolometer's spectral response, in the first case using a filtered blackbody and in the second using an ultrabroadband, THz photomixer, referred to a Golay cell, agree within 5%. Evidence of frequency-dependent scattering from ordinary clothing material, distinct from simple linear attenuation, is presented from an idealized laboratory experiment. However, the scattering is relatively weak, and unlikely to have a significant effect in practical THz imaging scenarios, particularly with ultrawide bandwidths.

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.

It has recently been suggested that near-field terahertz ionic contrast microscopy can be employed to image subtle changes in ionic concentrations arising from neuronal activity. To do so, however, requires that solvated ions exhibit significant absorbance at terahertz frequencies. The authors have investigated this issue and find that, at room temperature, the molar extinctions of both sodium chloride and guanidine hydrochloride are approximately two orders of magnitude below some previous measurements and are, therefore, too low to support the proposed imaging application.

We report the first experimental results for noise-equivalent power (NEP) and noise-equivalent temperature difference (NETD) of single-crystal ErAs:InAlGaAs , zero-bias rectifier diodes coupled to free space quasi-optically in the THz region. At a frequency of 639 GHz, an optical NEP of 4.0×10 −12 W / Hz 1/2 is measured with the rectifier coupled to a quasi-plane-wave coherent source through a single-turn square spiral antenna. With a broadband thermal (hot water) source, an NETD of 120 mK is measured from the same device. Antenna radiation patterns at 100 GHz and 639 GHz are also presented.

This paper describes a compact, self-complementary square-spiral antenna on a GaAs substrate with a broadside high-directivity (similar to 29 dB) frequency-independent pattern when coupled through a silicon hyperhemisphere. The driving-point resistance undulates between similar to 100 and 300 Omega from 200 GHz to 1 THz-much higher than the 72 Omega value from Booker's modified formula, but quite beneficial for coupling to high-impedance broadband devices. (c) 2006 Wiley Periodicals, Inc.

We report experimental results for the optical responsivity and noise-equivalent power, (NEP) of quasi-optically coupled, room-temperature ErAs-InGaAlAs rectifier diodes. Four-micron-diameter diodes were flip-chip coupled to self-complementary log-periodic and square-spiral antennas, and characterized with a 104-GHz Gunn diode oscillator coupled to the rectifiers through Variable attenuators, a feedhorn, an aspherical polymeric lens, and a Si hyperhemisphere. The log-periodic mounted device displayed a responsivity and specific NEP' of 0.9x10(3) V/W and 1.2x10(-12) W/Hz(1/2), respectively. The square-spiral mounted device displayed a responsivity and NEF of 1.2x10(3) V/W and 2.0x10(-12) W/Hz(1/2), respectively. All values were measured at a post-detection center frequency of 33 Hz.

We report a new photoconductive switch having an average output power of 44 microW, an instantaneous bandwidth of approximately 300 GHz, an output pulse width of approximately 2.2 ps, a peak output power of approximately 1.0 W, and an optical-to-electrical conversion efficiency of approximately 0.5% when pumped by a palm-sized mode-locked free-space laser at lambda=780.6 nm with an average power of 8.7 mW and an optical pulse width of approximately -230 fs. The switch is made from an ErAs:GaAs epitaxial layer inside a resonant optical cavity and coupled to a planar three-turn square spiral antenna.

A frequency domain transceiver is implemented with a pair of 780-nm-based ErAs:GaAs photomixers, one serving as the transmitter and the other as the receiver. The transceiver is all solid state, operated at room temperature, and has a fast response. The receiver detects the amplitude of the incoming electric field coherently and the sensitivity is limited by 1/f mechanisms which are excited by a photovoltage that is always present in the receiver photomixer. This letter reports the first measurements and calculations of the signal-to-noise (S/N) ratio in the transceiver. Measurement yields a differential terahertz power-to-current sensitivity of 4 mA/W and noise equivalent power of 0.1 pW/Hz. The maximum S/N ratio was 58 dB/rtHz around 86 GHz which is 17 dB lower than our theoretical estimate. The discrepancy is attributed to radiative coupling losses and nonoptimal laser spatial overlap. (C) 2005 American Institute of Physics.

We report the measurement of exceptionally bright, incoherent radiation in the THz region by frequency down-conversion of amplified spontaneous emission around 775-nm wavelength. The down-conversion technique is optical mixing in an interdigital photoconductive capacitor made from ultra fast ErAs:GaAs. The brightness temperature into a single spatial mode is approximately 1.1×10 5 K , making the new radiation at least 70 times more intense than common incandescent sources in the THz region.