KEYWORDS: Principal component analysis, Sensors, Signal to noise ratio, Terahertz radiation, Gallium arsenide, Signal detection, Antennas, Superlattices, Terahertz spectroscopy, Spectroscopy
Terahertz (THz) spectroscopy with high sensitivity is essential for biological application considering the strong absorption and scattering effects therein. As the most commonly used THz detector, the photoconductive antenna’s (PCA) response greatly relies on the properties of the substrate’s material. THz detection properties of the PCAs fabricated on low-temperature-grown GaAs (LT-GaAs) and ErAs:GaAs superlattices were compared at the sub-THz band. The detection efficiency of the PCAs with regard to incident laser power was characterized. In addition, using the PCAs as detectors, the signal-to-noise ratio (SNR) and dynamic range (DR) of a terahertz time-domain spectroscopy were quantified. The result indicates that the PCA detector with LT-GaAs has higher efficiency than the one with ErAs:GaAs. Consequently, the corresponding THz spectrometer has better SNR and DR. This result is contrary to the previous report, in which enhanced detection efficiency was observed with ErAs:GaAs-based PCA, which is probably due to the different structures of ErAs:GaAs superlattices used in the experiment.
Terahertz (THz) time-domain spectroscopy systems permit the measurement of a tissue’s hydration level. This feature makes THz spectrometers excellent tools for the noninvasive assessment of skin; however, current systems are large, heavy and not ideal for clinical settings. We previously demonstrated that a portable, compact THz spectrometer permitted measurement of porcine skin optical properties that were comparable to those collected with conventional systems. In order to move toward human use of this system, the goal for this study was to measure the absorption coefficient (μ a ) and index of refraction (n ) of human subjects in vivo. Spectra were collected from 0.1 to 2 THz, and measurements were made from skin at three sites: the palm, ventral and dorsal forearm. Additionally, we used a multiprobe adapter system to measure each subject’s skin hydration levels, transepidermal water loss, and melanin concentration. Our results suggest that the measured optical properties varied considerably for skin tissues that exhibited dissimilar hydration levels. These data provide a framework for using compact THz spectrometers for clinical applications.
Terahertz time-domain spectroscopy (THz-TDS) systems are capable of detecting small differences in water
concentration levels in biological tissues. This feature makes THz devices excellent tools for the noninvasive assessment
of skin; however, most conventional systems prove too cumbersome for limited-space environments. We previously
demonstrated that a portable, compact THz spectrometer permitted measurement of porcine skin optical properties that
were comparable to those collected with conventional systems. In order to move toward human use of this system, the
goal for this study was to collect the optical properties, specifically the absorption coefficient (μa) and index of refraction
(n), of human subjects in vivo. Spectra were collected from 0.1-2 THz, and measurements were made on the palm,
ventral (inner) and dorsal (outer) forearm. Prior to each THz measurement, we used a multiprobe adapter system to
measure each subject’s skin hydration levels, transepidermal waterloss (TEWL), skin color, and degree of melanin
pigmentation. Our results suggest that the measured optical properties were wide-ranging, and varied considerably for
skin tissues with different hydration and melanin levels. These data provide a novel framework for accurate human
tissue measurements using THz spectrometers in limited-space environments.
Microfluidic devices have been widely used in manipulation and analysis of individual cells in small-volume solutions. It
could be potentially used for studies of the interaction of THz radiation with biomolecules and cells in aqueous media.
We present a prototype microfluidic device that can be used for controlled cellular exposures to THz radiation. The
device is made of a PDMS microfluidic channel on glass substrate and consists of electrodes for cell concentration.
Initial cell concentration and THz transmission measurements have been performed on various prototype samples. Our
results demonstrate the feasibility of using microfluidic chips for potential “Lab-on-a-Chip” THz applications.
Various all-dielectric electromagnetic crystal (EMXT) based THz components, including filter/reflector, waveguide,
antenna, and transition structure to planar circuits are proposed and simulated. Several of them have been fabricated via
a THz rapid prototyping technique, and the measurements show very good consistency with the simulations. Potential
integrated THz micro-systems could be constructed using these components. The layer-by-layer printing virtue of the
rapid prototyping technique may enable the integration and packaging of various THz components in a systematic
manner.
We are exploring the degree to which one can control the spectral emission of heated photonic crystals (or, more
generally, electromagnetic crystal) structures in the THz frequency range. Because THz frequencies are well below the
room temperature thermal emission maximum, this configuration may realize a low power but extremely low cost
incoherent broadband THz source. Electromagnetic crystals are structures whose periodicity either enhances or reduces
the associated photonic density of states over some frequency range. Consequently, they either enhance or reduce its
thermal emission over the same frequency range. Thermal radiation from electromagnetic crystals has been studied
theoretically and experimentally for higher frequency ranges, but usually for infinite lattices. We have experimentally
and theoretically investigated a simple 1D, bi-layered electromagnetic crystal structure composed of air and silicon slabs.
We have calculated the emissivity using Kirchhoff's thermal radiation law, as well as by calculating the density of states
directly, and have compared successfully those results to the experimental values. Our ultimate goal is to be able to
control the spectral emission of an electromagnetic crystal in the THz region (or other wavelength ranges, such as the
infrared) by engineering its band structure. Controlled thermal emission, i.e., thermal management, could be used for
applications as diverse as solar energy convertors, thermoelectric devices, and integrated circuits.
Recent advances in rapid prototyping technologies have resulted in build-resolutions that are now on the scales
required for direct fabrication of photonic structures in the gigahertz (GHz) and terahertz (THz) regimes. To
demonstrate this capability, we have fabricated several structures with 3D bandgaps in these spectral regions.
Characterization of the transmission properties of these structures confirms the build accuracy of this fabrication
method. The result is a rapid and inexpensive fabrication technique that can be utilized to create a variety of
interesting photonic structures in the GHz and THz. We present the results of our characterization experiments
and discuss our current efforts in extending the technique to fabrication of other structure types.
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