Detection of concealed threats is a key issue in public security. In short range applications, passive imagers operating at
millimeter wavelengths fulfill this task. However, for larger distances, they will suffer from limited spatial resolution.
We will describe the design and performance of 0.8-THz imaging radar that is capable to detect concealed objects at a
distance of more than 20 meter. The radar highlights the target with the built-in cw transmitter and analyses the returned
signal making use of a heterodyne receiver with a single superconducting hot-electron bolometric mixer. With an
integration time of 0.3 sec, the receiver distinguishes a temperature difference of 2 K at the 20 m distance. Both the
transmitter and the receiver use the same modified Gregorian telescope consisting from two offset elliptic mirrors. The
primary mirror defines limits the lateral resolution of the radar to 2 cm at 20 m distance. At this distance, the field of
view of the radar has the diameter 0.5 m. It is sampled with a high-speed conical scanner that allows for a frame time
less than 5 sec. The transmitter delivers to the target power with a density less than ten microwatt per squared centimeter,
which is harmless for human beings. The radar implements a sensor fusion technique that greatly improves the ability to
identify concealed objects.
Being motivated by the possibility of fingerprinting and detecting VX nerve agent, we have investigated its stimulant,
i.e. malathion vapor, which is less toxic and commercially available, in the far-infrared/THz transition region and THz
frequency range. Such a spectroscopic study was carried out by using Fourier transform infrared spectroscopy (FTIR).
Our intention is to obtain a specific spectroscopic signature of VX nerve agent as a chemical warfare agent. Following
our experimental result, we have successfully observed eleven new absorption peaks from malathion vapor in the
spectral ranges from 15 cm-1 to 68 cm-1 and from 75 cm-1 to 640 cm-1. Specifically, in the far-infrared/THz transition
region, we have observed eight peaks and whereas in the THz region we have identified three relatively weak transition
peaks. In addition, we have investigated the dependence of the absorption spectra on temperature in the range from room
temperature to 60°C. In both of the frequency ranges, we have found that absorption coefficients significantly increase
with increasing temperature. By comparing the transition peaks in the two frequency ranges, we have concluded that the
frequency range of 400-640cm-1 is an optimal range for fingerprinting this chemical specie. We have designated two
peaks for effectively and accurately identifying the VX nerve agents and one peak for differentiating between malathion
and VX nerve agent.
We report on THz spectra of RDX obtained from various domestic and international sources. The observed spectral
differences can be traced to the method of RDX manufacture. Depending on the method of manufacture, the resulting
energetic material will contain imperfections within the crystal such as voids, solvent trapped within the voids,
crystalline dislocations, explosive mixtures and co-crystallization of other energetic byproducts. Additionally, neat
energetic material crystallites often range from tens to hundreds of microns. To investigate these phenomena,
transmission and reflection mode THz spectroscopy was performed. The resulting spectral differences are interpreted in
an attempt to identify material and contaminant effects.
We present a novel technique for detecting objects that are concealed by textiles. Items of virtually all materials
can be sensed, which includes metals as well as dielectrics. Our technique detects the acoustic phase lag of
objects when they follow a driven oscillation. The acoustic phase is measured interferometrically with continuous
wave radiation at about 77 GHz. Typical oscillation amplitudes are about 1 μm, which is close to the human
perception level. The technique provides no insight into the concealed item's material properties. It exclusively
shows whether or not an object is hidden, which is the most relevant information for many sensing applications.
Many terahertz imaging systems under development will be employed in outdoor environments, where spatial and
temporal fluctuations of atmospheric absorbing species can affect image quality. Absorption across most of the terahertz
band is dominated by water vapor. Active systems that illuminate targets with scanned ("flying spot") or floodlight
terahertz sources will experience some spatial and temporal noise modulation of target plane irradiance due to path-integrated
inhomogeneities in the turbulent water vapor density field. We have analyzed data collected during field
measurement campaigns conducted at the White Sands Missile Range during the spring and summer of 2007 for spectral
characteristics and diurnal variations of water vapor fluctuations under dry to moderately humid synoptic conditions.
The results of these analyses were then used to model the statistics of irradiance fluctuations that might be observed in
the target plane of a THz imager under varying propagation conditions. The measurements acquired can also be
compared with a statistical model of path-integrated absorptance considering either the evolution of the absorption with
time or the effects of decorrelation in absorber for two angularly separated lines-of-sight.
Recent advances in ultrafast optical laser technology have improved generation and detection of energy within the
terahertz (THz) portion of the electromagnetic (EM) spectrum. One promising application of THz spectroscopy
is the detection of explosive materials and chemical or biological agents. This application has been motivated by
initial measurements that indicate that explosives may have unique spectral characteristics in the THz region thus
providing a discernible fingerprint. However, since THz wavelengths are 10's to 100's of microns in scale, rough
interfaces between materials as well as the granular nature of explosives can cause frequency-dependent scattering
that has the potential to alter or obscure these signatures. For reflection spectroscopy in particular the measured
response may be dominated by rough surface scattering, which is in turn influenced by a number of factors
including the dielectric contrast, the angle of incidence and scattering, and the operating frequency. In this paper,
we present measurements of THz scattering from rough surfaces and compare these measurements with analytical
and numerical scattering models. These models are then used to predict the distortion of explosive signatures
due to rough surface interfaces with varying surface height deviations and correlation lengths. Implications of
scattering effects on the performance of THz sensing of explosive materials are presented and discussed.
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.
A terahertz frequency domain spectrometer is implemented using two ErAs:GaAs photomixers in a highly
compact configuration, utilizing all solid-state components and no moving parts. Digital signal processing
electronics provide precise frequency control and yield ~200 MHz accuracy of the THz signal frequency.
Continuous frequency sweeping is demonstrated with better than 1 GHz resolution from 200 GHz to 1.85
THz. The coherent detection sensitivity is shown to be in good agreement with previous theoretical
predictions and yields a signal-to-noise ratio of 80 dB*Hz at 200 GHz and 60 dB*Hz at 1 THz through a path
length in air of one foot.
Terahertz imaging currently is done using single pixel imagers mechanically scanned over the field of view. Focal
planes will reduce the need for mechanical scanning but are still under development. In the 70s, Jacobs , et. al.,
proposed and demonstrated a device for millimeter wave imaging using a single pixel. The device obviated the need for
large mechanical scanning mechanisms by using an optically scanned bulk semiconductor in a resonant structure. In this
research, a device of this type is analyzed for suitability as an element in a terahertz or sub-millimeter wave imager. The
device is simulated under simultaneous illumination from a coherent RF source and a coherent optical source (a laser).
A computational electromagnetic model of the device is described. Device and system performance metrics are defined
and predicted performance presented. Finally design of a system incorporating this device is discussed.
Time domain terahertz (TD-THz) reflection imaging tomography can be used to investigate the laminar structure of
objects. In a monostatic configuration, a sequence of pulses is generated by reflection from each discontinuity in index
of refraction. Through analysis of the return pulses, the material absorption and index of refraction properties of each
layer can be determined. TD-THz reflection tomography can be used to precisely measure the thickness of coatings
such as yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) on aircraft engine turbine blades; paint on
aircraft, ships, and cars; and other thin film measurement applications. In each of these cases, precise determination of
the optical delay of the TD-THz pulses is required with as little as sub-10 femtosecond precision for pulses which can be
greater than 500 fs in duration. We present a method to accurately measure optical delay between layers where the
pulses are fit to a reference template. These are demonstrated to achieve micron scale accuracy in coating thickness. As
an example, TD-THz non destructive evaluation (NDE) imaging is used to two-dimensionally map the thickness of YSZ
TBCs on aircraft engine turbine blades. Indications of thermal degradation can be seen. The method is non-contact,
rapid, and requires no special preparation of the blade.
Terahertz (THz) cameras are expected to be a powerful tool for future security applications. If such a technology shall
be useful for typical security scenarios (e.g. airport check-in) it has to meet some minimum standards. A THz camera
should record images with video rate from a safe distance
(stand-off). Although active cameras are conceivable, a
passive system has the benefit of concealed operation. Additionally, from an ethic perspective, the lack of exposure to a
radiation source is a considerable advantage in public acceptance.
Taking all these requirements into account, only cooled detectors are able to achieve the needed sensitivity. A big leap
forward in the detector performance and scalability was driven by the astrophysics community. Superconducting
bolometers and midsized arrays of them have been developed and are in routine use. Although devices with many pixels
are foreseeable nowadays a device with an additional scanning optic is the straightest way to an imaging system with a
useful resolution. We demonstrate the capabilities of a concept for a passive Terahertz video camera based on
superconducting technology. The actual prototype utilizes a small Cassegrain telescope with a gyrating secondary
mirror to record 2 kilopixel THz images with 1 second frame rate.
Terahertz medical imaging has emerged as a promising new field because of its non-ionizing photon energy
and its acute sensitivity to water concentration. To better understand the primary contrast mechanism in
THz imaging of tissues, the reflectivity of varying water concentrations was measured. Using a pulsed THz
reflective imaging system, a 0.3 mm thin paper sample with varying water concentrations was probed and
from the measured data a noise equivalent delta water concentration (NEΔWC) of 0.054% was derived.
The system is based on a photoconductive pulsed source and time-gated waveguide-mounted Schottky
diode receiver. It operates at a center frequency of 500 GHz with 125 GHz of noise-equivalent bandwidth
and at a standoff of 4 cm, the imaging system achieved a spot size of 2.2 mm. The high water sensitivity of
this system was exploited to image burned porcine (pig) skin models in reflection using differences in
water content of burned and unburned skin as the contrast mechanism. The obtained images of the porcine
skin burns are a step towards the ability to quantify burn injuries using THz radiation.
This paper describes the design, implementation and measurements of a detector for imaging purposes at terahertz
frequencies. The detector comprises of a ring slot antenna coupled to a high temperature superconducting Josephson
Junction device. The detector was shown to respond to an incident field at 0.6 THz. An imaging system was constructed
to test the detector's ability to generate images at 0.6 THz. Images have been acquired that demonstrate the ability of the
detector to operate in an imaging mode in scenarios that exploit terahertz radiation's unique properties including
penetration through packaging, sensitivity of water and millimeter scale resolution.
The physics, the theory and the engineering formulas of 1/f noise in ErAs-based all-epitaxial Schottky diodes
are presented in a way related to the general quanum 1/f noise formulas developed by the author and van der Ziel ealier
for pn junctions, but with inclusion of the image force contribution of an electron at the metal-semiconductor interface.
On this base the phase noise introduced by mixers constructed with the ErAs Schottky diodes was also studied and can
now be calculated analytically with the Quantum 1/f effect formulas.
Optical systems for THz imaging frequently consist of readily available components - typically a single spherical or
parabolic surface. For THz imaging applications, the range is short, due to limitations associated with atmospheric
attenuation. For these applications, single spherical and parabolic surfaces are neither stigmatic, aplanatic nor Herschel.
As a result, many THz imaging systems exhibit significant image degradation caused by primary aberrations. Further,
the short range limitations frequently result in image degradation due to near violation of the paraxial assumption. For
improved imaging, an aplanatic system is required. To achieve aplanatism, a minimum of two aspheric surfaces is
required. Aplanatism requires stigmatic performance which dictates surfaces that are conic sections of revolution. A
minimum of two are required to exhibit stigmatism and meet the sine condition. An improvement of receiver form
factor allows for a decrease in optical image distance and an increase in system magnification factor. This significantly
improves a number of THz imaging characteristics such as depth-of-field while maintaining the diffraction-limit
resolution and reducing the primary objective diameter. Reduction of objective diameter reduces signal strength - principally at the expense of specular reflections. This paper summarizes the results of the optical system design and its
incorporation into THz imagers containing THz receivers with improved form factors. Efforts at incorporation of optical
zoom will be presented.
Here we apply the quantum theory of 1/f noise to evaluate the 1/f noise of double-barrier RTDs in
general, and to the phase noise of oscillators based on them. If the energy of the tunneling electrons is close
to the first energy level in the well, resonance occurs, and a peak I of the current through the diode occurs at
the voltage v. If the applied voltage increases further, only a negligibly small non-resonant current trickle i
remains at the valley voltage V>v. Scattering processes that reduce the energy of the carriers from the
resonant energy eV will always be present, generating a finite current minimum i at v. Their quantum 1/f rate
fluctuations cause the 1/f current fluctuations in i. The results are compared with experimental data on RTD
oscillators. The resulting phase noise of oscillators built on this basis is calculated with the quantum 1/f
A prototype of terahertz imaging system has been built in CSIRO. This imager uses a backward wave oscillator as the
source and a Schottky diode as the detector. It has a bandwidth of 500-700 GHz and a source power 10 mW. The
resolution at 610 GHz is about 0.85 mm. Even though this imaging system is a coherent system, only the signal power is
measured at the detector and the phase information of the detected wave is lost. Some initial images of tree leaves,
chocolate bars and pinholes have been acquired with this system. In this paper, we report experimental results of an
attempt to improve the resolution of this imaging system beyond the limitation of diffraction (super-resolution). Due to
the lack of phase information needed for applying any coherent super-resolution algorithms, the performance of the
incoherent Richardson-Lucy super-resolution algorithm has been evaluated. Experimental results have demonstrated that
the Richardson-Lucy algorithm can significantly improve the resolution of these images in some sample areas and
produce some artifacts in other areas. These experimental results are analyzed and discussed.