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.
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.
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.
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.
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.
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 0.9x103 V/W and
1.2x10-12 W/Hz1/2, respectively. The square-spiral mounted device displayed a responsivity and NEP' of 1.2x103 V/W
and 2.0x10-12 W/Hz 1/2, respectively. All values were measured at a post-detection center frequency of 33 Hz.