With increasing interest in wireless communications at terahertz (THz) frequencies, investigations on the link performance in indoor and outdoor environments are required. In order to analyze the signal impairments caused by outdoor weathers and indoor surface reflections, wireless channels are employed using a continuous wave (CW) signal at five discrete frequencies (100, 200, 300, 400 and 625 GHz) with a data rate of several Gb/sec. The link performance in rainy, snowy, and atmospheric turbulence weathers is analyzed by measuring the power and bit-errorratios (BERs). Scattering effects due to reflection by indoor rough surfaces are investigated. Predictions for power attenuation and link performance in these scenarios are conducted and compared with experimental results.
The Terahertz Time Domain Reflection Spectroscopy (THz-TDS) method of paint layer diagnostics is a non-contact
electromagnetic technique analogous to pulsed-ultrasound with the added capability of spectroscopic characterization.
The THz-TDS sensor emits a near-single cycle electromagnetic pulse with a bandwidth from 0.1 to 3 THz. This wide
bandwidth pulse is focused on the coating, and echo pulses are generated from each interface (air-coating, layer-layer,
coating-substrate). In this paper, the THz-TDS method is applied to specialty aircraft coatings. The THz-TDS method is
able to penetrate the whole coating stack and sample the properties of each layer. Because the reflected pulses from
individual layers typically overlap in time, the complex permittivity function and thickness of each layer is determined
by a best fit of the measured reflection (either in time or frequency domain) to a layered model of the paint. The THz-
TDS method is applied to specialty coatings prior to and during accelerated aging on a series of test coupons. The
coupons are also examined during aging using ATR (attenuated total reflectance)-FTIR spectroscopy, Raman scattering
spectroscopy, and Scanning Electron Microscopy (SEM) to ascertain, quantify, and understand the breakdown
mechanisms of the coatings. In addition, the same samples are characterized using THz-TDS techniques to determine if
the THz-TDS method can be utilized as a non-destructive evaluation technique to sense degradation of the coatings. Our
results suggest that the degradation mechanism begins in the top coat layer. In this layer, 254 nm UV illumination in
combination with the presence of moisture works partially with oxides as catalysts to decompose the polymer matrix
thereby creating porosity in the top coat layer. Since the catalytic effect is partial, loss of the oxides by chemical reaction
can also occur. As the topcoat layer becomes more porous, it allows water vapor to permeate the topcoat layer and
interact with the rain erosion layer via carbonization of the polymer matrix in the rain erosion layer. The presence of the
salt accelerates the pitting degradation. The goal of this paper is to determine if THz-TDS can be used to sense
degradation of the coating.
A RF wireless capacitive pressure sensor is developed. The sensor has integrated inductor with the pressure sensitive
capacitor as LC circuit. The resonant frequency of the sensor changes as the capacitance changes with applied pressure.
The sensor uses LPCVD silicon nitride as sensitive membrane and the residual stress of the membrane has been measure
as 139MPa. The sensor has size of 10 mm × 4 mm × 0.5 um. The sensor presents a pressure sensitivity of 1.65
MHz/cmH2O in pressure range of 0-20 cmH2O. The deflection of different shape of membranes is discussed. The
deflection of square membrane is 130% to circular membrane under same applied pressure.
A RF wireless capacitive flow sensor is developed. The sensor has integrated inductor with the flow sensitive capacitors
as LC circuit. The resonant frequency of the sensor changes as the capacitance changes with applied flow. The sensor
uses LPCVD silicon nitride as sensitive membrane and the residual stress of the membrane has been measure as 139
MPa. The sensor has size of 10 mm × 4 mm × 0.5 μm. The sensor integrated two pressure sensors together and designed
related to flow 5-20ml/hour. The deflection of different shape of membranes and the parameters of flow sensor
sensitivity are discussed. The deflection of square membrane is 130% to circular membrane under same applied
Experimental results of homodyne terahertz interferometric 2-D imaging of RDX are presented. Continuous waves at
0.25-0.6 THz are used to obtain images of a C-4 sample at several THz frequencies. The performance of an N element
detector array is imitated by only one detector placed at N positions. The distance between the C-4 sample and the
detector array is ~30 cm. By taking interferometric images at several THz frequencies RDX can be recognized by the
spectral peak at 0.82 THz. Simulations of interferometric images of two point sources of spherical waves are presented.
The terahertz interferometric imaging method can be used in defense and security applications to detect concealed
weapons, explosives as well as chemical and biological agents.
The meantime between failures of the thin film strain sensor is a critical indicator for
future U.S. Army field sensing application . This accelerated lifetime aging test would
characterize the existing flexible strain sensor for repeated load response/application. A
typical industrial maximum number of testing cycles used are about 10x106 cycles .
Experimental results of two-dimensional homodyne terahertz interferometric imaging are presented. The
performance of an N element detector array is imitated by only one detector placed at N positions.
Continuous waves at 0.25-0.3 THz are used to detect concealed objects: a metal object and an RDX
sample. The terahertz interferometric imaging method can be used in defense and security applications to
detect concealed weapons, explosives as well as chemical and biological agents.
Experimental results of homodyne terahertz interferometric 1-D and 2-D imaging are presented. Continuous waves at 0.25-0.3 THz are used to detect a metal object behind a barrier. The performance of an N element detector array is imitated by only one detector placed at N positions. The reconstructed images are in a good agreement with theoretical predictions. The terahertz interferometric imaging method can be used in defense and security applications to detect concealed weapons, explosives as well as chemical and biological agents.
In recent times, the far infrared or the terahertz (1 THz = 1012 cycles/sec and 300μm in wavelength) region of electromagnetic spectrum has become a promising radiation for spectroscopic identification of different types of biomaterials. The present work investigates the effect of grain size on the THz spectra of chalk, salt, sugar and flour using THz time-domain spectroscopy. It has been observed that at lower frequencies, solids of small grain sizes of nonabsorbing materials show rising trends in their extinction spectra. Here, we obtain extinction spectra of granular salt, chalk, sugar and flour between 0.2 to 1.2 THz and show that the experimentally obtained extinction can be predicted on the basis of the Mie Scattering model for small grain sizes. The current study is an attempt to understand the absorption spectrum of a few such materials having no significant intrinsic absorption in the THz region by separating the independent contributions of true absorption of the material and scattering losses due to its morphology in the extinction of the material. This would help in distinguishing these materials based on their rising trend of the extinction spectra at lower frequencies.
In recent times, Terahertz (1 THz = 1012 cycles/sec and 300 μm in wavelength) spectroscopy has become a promising technique for spectroscopic identification of different materials having contemporary interest. In this study we report a direct measurement of reflection spectra of the explosive C-4, which shows significant absorption around 0.8 THz, using THz time domain spectroscopic techniques. A contrast in reflection of around 8% has also been observed between the neighboring frequencies of 0.7 THz and 0.9 THz. The spectral data have been used to create realistic synthetic images for use in simulations of interferometric detection in a stand-off THz imaging system. The results obtained are analyzed using Artificial Neural Networks for positive identification of the agents with an interferometric array of few linear detectors in near field mode.
In recent times, terahertz (THz) or the far-infrared region of the electromagnetic spectrum has gained critical significance due to many potential applications including medical diagnostics, nondestructive evaluation of material parameters, chemical sensing, remote sensing and security screening. However with the development of various applications, the need of guided systems for the transmission of THz radiation have posed a challenge, as a flexible waveguide could simplify the propagation and detection of THz waves in remote locations without atmospheric absorption. Different structures, such as, rigid hollow metallic waveguides, solid wires, or short lengths of solid-core transparent dielectrics such as sapphire and plastic have already been explored for THz guiding to characterize their individual loss and dispersion profile. Recently, it has been reported that copper coated flexible, hollow polycarbonate waveguide has low loss of less than 4 dB/m with single mode operation at 1.89 THz. In the present study, using a broadband THz source of photoconductive antennae, we characterize the loss and dispersion profile of hollow core polycarbonate metal waveguides in the frequency range of 0.2 to 1.2 THz.
Recently, there has been a significant interest in employing Terahertz (THz) technology, spectroscopy and imaging for standoff detection applications. There are three prime motivations for this interest: (a) THz radiation can detect concealed weapons since many non-metallic, non-polar materials are transparent to THz radiation, (b) target compounds such as explosives, and bio/chemical weapons have characteristic THz spectra that can be used to identify these compounds and (c) THz radiation poses no health risk for scanning of people. This paper will provide an overview of THz standoff detection of explosives and weapons including discussions of effective range, spatial resolution, and other limitations. The THz approach will be compared to alternative detection modalities such as x-ray and millimeter wave imaging.
It has been suggested that interferometric/ synthetic aperture imaging techniques, when applied to the THz regime, can provide sufficient imaging resolution and spectral content to detect concealed explosives and other weapons from a standoff distance. The interferometric imaging method is demonstrated using CW THz generation and detection. Using this hardware, the reconstruction of THz images from a point source is emphasized and compared to theoretical predictions.
The application of near-field interferometric imaging to the Terahertz frequency range for detection of concealed objects is discussed. A circular array architecture can be employed to compensate for near-field distortions and increase the field of view and depth of focus. The lateral and focusing errors of this imaging method are discussed as well as the trade-offs of interferometric imaging compared to a focal plane array architecture.
Ultrafast laser techniques have opened up a tremendous research opportunity in studying the interaction of short pulses of light with matter. With discovering of the picosecond photoconducting hertzian dipoles and high-brightness THz beams characterized with an ultrafast detector, we have seen more and more applications of ultrafast light in non-invasive imaging. Standard methods, when applied to the measurement of thin optical materials, doesnot independently determine the material's thickness and index of refraction. The proposed method is fundamentally different from other imaging such as contrast difference in optical coherent tomography (OCT) or the peak-to-peak intensity ratio as in THz imaging to determine index of refraction and thickness. We show that the application of ultrafast techniques allows simultaneous measurements of material thickness and optical constants in optical precision from transmission measurements. Such finding invites new perspectives in imaging and other applicable disciplines such as imaging processing after recording of the THz waveform of biological samples.
This article shows the applications of ultrafast light in studying material optical properties and its application for rudimental imaging. Standard methods, when applied to the imaging, can not independently determine the material's thickness and index of refraction. The proposed method is fundamentally different from other imaging such as contrast difference in optical coherent tomography (OCT) or the peak-to-peak intensity ratio as in THz imaging to determine index of refraction and thickness. We show that the application of ultrafast techniques allows simultaneous measurements of material thickness and optical constants in optical precision from transmission measurements. Such finding invites new perspectives in imaging and other applicable disciplines.
Terahertz Time domain spectroscopy (THz-TDS) can provide the optical response of a medium in both amplitude and phase. We show that such capability can enable a detail analysis of optical properties of biological sample. Such study is important for standoff detection of presence of biological sample, where a detail analysis is difficult if not possible due to a complicated system involved and multiple effects involved. We proposed a transfer function study of the response of such system.
Terahertz Time domain spectroscopy (THz -TDS ) can provide the optical response of a medium in both amplitude and phase. We show that such capability can enable a detail analysis of optical properties of RDX sample. Such study is important for standoff detection of presence of RDX sample, where a detail analysis is difficult if not possible due to a complicated system involved and multiple effects involved. We proposed a match filter method for detection of RDX inside or behind a barrier.
Terahertz Time domain spectroscopy (THz -TDS ) can provide the optical response of a medium in both amplitude and phase. We show that such capability can enable a detail analysis of optical properties around a resonance regime. Such study is important for standoff detection of explosive material where numerous absorption peaks exist. We proposed a model where one can synthesize the optical properties with THz-TDS around the resonance regimes.
The imaging properties of planar, spherical, and circular interferometric imaging arrays are examined in the near-field region limit. In this region, spherical and circular array architectures can compensate for near-field distortions and increase the field of view and depth of focus. The application of near-field interferometric imaging to the Terahertz frequency range for detection of concealed objects is emphasized.
A non-invasive means to detect and characterize concealed agents of mass destruction in near real-time with a wide field-of-view is under development. The method employs spatial interferometric imaging of the characteristic transmission or reflection frequency spectrum in the Terahertz range. However, the successful (i.e. low false alarm rate) analysis of such images will depend on correct distinction of the true agent from non-lethal background signals. Neural networks are being trained to successfully distinguish images of explosives and bioagents from images of harmless items. Artificial neural networks are mathematical devices for modeling complex, non-linear relationships. Both multilayer perceptron and radial basis function neural network architectures are used to analyze these spectral images. Positive identifications are generally made, though, neural network performance does deteriorate with reduction in frequency information. Internal tolerances within the identification process can affect the outcome.
We have conducted visible pump-THz probe experiments on single wall carbon nanotubes (SWCNTs) deposited on quartz substrates. Our results suggest that the photoexcited nanotubes exhibit localized transport due to Lorentz-type photo-induced localized states. These experiments were repeated for ion-implanted, 3-4nm Si nanoclusters in quartz for which a similar behavior was observed.
A proposed, non-invasive, means to detect and characterize concealed biological and explosive agents in near real-time with a wide field-of-view uses spatial imaging of their characteristic transmission or reflectivity wavelength spectrum in the Terahertz (THz) electro-magnetic range (0.1-3 THz). Neural network analyses of the THz spectra and images will provide the specificity of agent detection and reduce the frequency of false alarms. Artificial neural networks are mathematical devices for modeling complex, non-linear functionalities. The key to a successful neural network is adequate training with known input-output data. Important challenges in the research include identification of the preferred network structure (e.g. multi-layer perceptron), number of hidden nodes, training algorithm (e.g. back propagation), and determination of what type of THz spectral image pre-processing is needed prior to application of the network. Detector array images containing both spectral and spatial information are analyzed with the aid of the Neurosolutions(TM) commercial neural network software package.
We have conducted visible pump-THz probe experiments on single wall carbon nanotubes (SWCNTs) on quartz substrates. Our results suggest an upper limit to the carrier-lifetime, which is on the order 1.5ps, limited only by the THz pulse duration. These experiments were repeated for ion-implanted, 3-4nm Si nanoclusters in quartz for which the carrier lifetime was also assessed at 1.5ps. THz time-domain spectroscopy (THz-TDS) of SWCNTs revealed that the THz pulse peak transmission changed under optical illumination.
Persistent photoconductivity and photoinduced superconductivity have been previously observed in various stoichiometries of YBa2Cu3Ox. The physical mechanism which is responsible for these effects is still under debate. Through an investigation of laser ablated YBa2Cu3Ox thin film compositions, photoluminescence spectra and infrared spectroscopy, the possibility that the mechanism may be defect related has been supported. A correlation between photoluminescence spectra and wavelength dependence support an oxygen defect model of photoinduced persistent conductivity and superconductivity in which the oxygen vacancies act as weakly luminescent F-centers and F+-centers under illumination. Upon infrared illumination the trapped electrons are photoexcited resulting in a partial quenching of the persistent photoconductivity state.
We investigated the gain and noise figure dependence on signal wavelength for erbium doped fiber amplifiers (EDFA) counter-pumped at 980 nm. Using a two level modeling technique, we found a good agreement between our experimental measurements and the corresponding theoretical predictions for EDFAs operating under or far from full population inversion. Finally, we describe design trade-offs between small signal gain and noise figure for a range of active fiber length EDFAs at wavelengths of 1531 nm and 1548 nm.