Far infrared spectra of 14 commonly used explosive samples have been measured by using Fourier Transform Infrared Spectroscopy (FTIR) and THz Time-Domain Spectroscopy (THz TDS). New absorption resonances between 20 cm-1 and 650 cm-1 are reported. Below 20 cm-1, no clear absorption resonances are observed in all the explosives. There is a good consistency of far-IR spectrum measured by Far-FTIR and by THz TDS in explosives 3,5-DNA and 2,4-DNT. Observed far-IR spectrum of TNT is compared with a previously reported theoretical calculation.
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.
In recent years our group has made significant progress toward the goal of a scalable, inexpensive terahertz imaging system for the detection of weapons concealed under clothing. By actively illuminating the subject under examination with only moderate source power (few milliwatts) the sensitivity constraints on the detector technology are significantly lessened compared to purely passive millimeter-wave detection. Last year, we demonstrated a fully planar, optically lithographed, uncooled terahertz imaging array with 120 pixels on a silicon substrate 75 mm in diameter. In this paper we present the recent progress on improving the responsivity of the individual microbolometers by a simple technique of surface micromachining to reduce the substrate thermal conduction. We describe the microbolometer array fabrication and present results on devices with a measured electrical responsivity of over 85 V/W (electrical NEP ~25 pW/rtHz), an improvement by a factor of two over current substrate-supported bolometers.
This work presents spectroscopic characterization results for biological simulant materials measured in the terahertz gap. Signature data have been collected between 3 cm-1 and 10 cm-1 for toxin Ovalbumin, bacteria Erwinia herbicola, Bacillus Subtilis lyophilized cells and RNA MS2 phage, BioGene. Measurements were conducted on a modified Bruker FTIR spectrometer equipped with the noise source developed in the NRAL. The noise source provides two orders of magnitude higher power in comparison with a conventional mercury lamp. Photometric characterization of the instrument performance demonstrates that the expected error for sample characterization inside the interval from 3 to 9.5 cm-1 is less then 1%.
Pulsed THz imaging is a promising non-destructive technology based on its high transmission through selected dielectric materials and its capability to provide time-of-flight and spectral information. The traditional method of the pulsed THz imaging is a point-to-point reflective scanning system. The image is acquired by analyzing the peak amplitude information of the THz pulse in the time-domain at each pixel. It requires the THz beam or sample scanned. In this paper, we present our approach of large scale, focal plane THz wave imaging. In our 2-D focal plane THz wave imaging, the THz beam is expanded to be 60 mm in diameter. The THz beam illuminates the target in a reflective manner, in which a polyethylene lens projects the image onto a 40 mm by 40 mm by 2 mm ZnTe sensor crystal. The probe beam is expanded to be 40 mm in diameter and overlap with the THz beam on the sensor. The modulated probe beam profile carrying the image information is captured by a CCD camera. This technique enables us to view the objects which are optically opaque but transparent in THz frequency and shows feasibility in remote sensing, security inspection, and military defense applications.
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.
Terahertz imaging and spectroscopy are being studied for inspecting packages and personnel, but advanced THz sources with much greater power are needed to increase the signal-to-noise ratio, and much greater frequency bandwidth to obtain more information about the target. Photomixing in resonant laser-assisted field emission is a new method that shows potential for increasing both the output power and frequency bandwidth by more than an order of magnitude. Tunneling electrons have a resonance with a radiation field, so a highly focused laser diode (670 nm, 30 mW) increases the emitted current enough to be seen with an oscilloscope, in good agreement with simulations. The electron-emitting tip is much smaller than optical wavelengths, so the surface potential follows each cycle of the incident radiation. Electron emission responds to the electric field with a delay τ< 2 fs, and the current-voltage characteristics of field emission are highly nonlinear. Thus, photomixing in laser-assisted field emission can cause current oscillations that may be tuned from DC to 500 THz (1/delay). A field emission current density of 1012 A/m2 can be generated using a 20 pJ 70 fs pulse from a Ti:sapphire laser, to provide 200 W THz pulses. Microwave prototypes for 1-10 GHz are now being tested.
We discuss several tradeoffs presented in the design of active imaging systems for the 100 to 1000 GHz frequency range, describe how we have addressed them in the design of a scanning, 95 GHz, bolometer-based imager for concealed weapons detection that is nearing completion, and describe how the system architecture can be modified to scale the operating frequency to the 650 GHz atmospheric window.
In this work, we present initial results of time-domain Fourier transform spectroscopy in the THz frequency range (from 1 to 20 THz) to detect and identify explosives and related compounds (ERCs). Comparison is made with conventional FT-IR spectroscopy. A comparison can also be make with the results of chemical modeling calculations (reported previously) to obtain spectroscopic information on ERCs and environmental background.
Terahertz (T-rays) spectroscopy has recently emerged as a powerful method to access a heretofore barely explored region of the electromagnetic spectrum where fundamental molecular resonances occur. Besides their importance for fundamental research, these resonances could be used as signatures in the identification of molecular species and as sensitive probes in a wide variety of molecular processes.
In this paper we consider the potential of THz spectroscopy in the application to relevant biomedical and homeland security problems such as the analysis of normal and diseased tissues and the detection of toxic biomolecules.
As examples, we present preliminary experimental data which suggest that THz spectroscopy: 1) can discriminate between cancerous and normal tissue, and 2) can reveal the presence of foreign substances hidden in an envelope and even allow their specific identification. This capability is of particular relevance as a straightforward homeland security tool for the detection of anthrax and other biotoxic molecules.
We review dynamic simulations on a molecular device named "nanocell" proposed for THz signal processing. In this preliminary study we conclude that signals applied to nanosized gold clusters interconnecting single molecules can modulate vibrational modes in the THz spectrum of the internal coordinates defining molecular bonds. Intensities involved in natural vibrational modes, which are experimentally recoverable in spectroscopy measurements. We can recover components due to couplings between local modes, and thus we provide a computational simulation of the possibility of using molecular vibrational modes for molecular electronics. The vibration of atoms can encode information that reflects local variations of electrical dipole and polarizability.
Using the special dispersion properties of photonic crystals (PhCs), we present a promising novel coupling device, the terahertz (THz) planar photonic crystal (PhC) lens. Three-dimensional finite-difference time-domain (3D-FDTD) calculations show a 90% power transfer from a 100 mm waveguide to a 10 mm waveguide, and experimental results confirm its high efficiency. Furthermore, the PhC lens couples the wave into a PhC line-defect waveguide is also reported. These achievements manifest the usefulness of the PhC lens as an effective approach to couple the wave into future THz circuits.
In this paper we demonstrate a superconducting bolometer, coupled to a lithographic antenna. The detector is operated at 4.2 K, and has an electrical noise equivalent power (NEP) of 14 fW/Hz1/2. We note that this sensitivity is approaching the typical background noise limit for terrestrial observations. The attractive feature of antenna-coupled microbolometers is that the simple fabrication procedure allows straightforward scaling to arrays, multi frequency capability, as well as intrinsic polarization selectivity. These features potentially enable the remote detection of chemical or biological agents by measuring differential absorption with two or more bolometers coupled to antennas designed for the intended frequencies. The noise equivalent temperature difference attainable with these detectors is around 40 mK at 0.5 THz for an integration time of 1/30 s and 30 % bandwidth, which would enable unprecedented image quality in application to detection of concealed weapons. Although the devices require cooling to cryogenic temperatures, we note that compact, closed-cycle cryocoolers exist on the market, which removes the need for liquid cryogens and provides user-friendly operation.
Conventional lenses are important components for many terahertz applications, but ordinary lenses are very difficult to fabricate for short-focal lengths. Multi-level phase-corrected zoned lens antennas have been investigated with particular application at terahertz wavelengths. These zoned lenses (or diffractive optics) give better performance than ordinary lenses, and because of their planar construction are easier and cheaper to fabricate. The depths of cut needed for a grooved zone plate are quite small, even when materials with low dielectric constants are used. Zoned lenses have been built and tested at various frequencies from 100 GHz to 1.5 THz, with phase correction levels of half-wave, quarter-wave, or eighth-wavelength. The inherent losses in transparent materials increase monotonically over this frequency range. Typical low-loss materials include polystyrene, polyethylene, Teflon, polycarbonate, polystyrene foam, foamed polyethylene, low density polytetrafluoroethylene (PTFE), TPX, quartz, sapphire, and silicon. Low dielectric-constant materials are normally preferred to reduce reflection and attenuation losses. Techniques for cutting or milling the materials to small dimensions are important, because at 1.0 THz an eighth-wavelength correction for silicon is only 15 μm. Another characteristic of zoned diffraction optics is their frequency behavior. Previous investigations have considered their bandwidth dependence and quasi-periodic extended frequency response for a specified focal length. As frequency changes, the focal point moves along the axis of the zoned lens. An analysis is given to explain this effect.
There is a critical need throughout DoD, the U.S. government, and the commercial sector for cost-effective monitoring systems to detect airborne biological warfare (BW) agents. At present, solutions for this sensor need are relatively expensive and have a high false alarm rate. Manning Applied Technology is developing a compact, portable bioaerosol sampling system for continuous monitoring of air quality, both at field locations and fixed installations. The instrument is premised on optical interrogation via a multi-step process. The first step is electrostatic concentration, to improve detection limits. An advantage of electrostatic particle concentration is the power efficiency, relative to impactors, cyclones and filter-based systems. The second step is presentation for particle analysis, which would employ one of several unique FT spectrometer designs. The advantage of spectroscopic interrogation of bioaerosol particles is the very low cost of each analysis, with no consumables required. It is thought that mid-IR and THz frequency ranges offer the best potential for accurate discrimination. The third, optional step, is archiving the collected particles for further analysis. To reduce component costs in the Fourier transform spectrometer, an optical replication process has been developed and tested, with promising results. The replication and optical testing methods are described in detail.
We report on development of a turn-key, cryogen-free bulk p-Ge laser which is broadly tunable over the range 1.5 to 4.2 THz. A 4 K closed cycle refrigerator was used to eliminate the need for liquid cryogen. A SmCo permanent magnet assembly provides the necessary magnetic field for the laser. A customized high voltage (HV) power supply and
thyratron pulser were developed to replace the stack of general electronics previously used to operate the laser.
Multi-layer mirrors capable of >99.9% reflectivity at ~100 micron wavelengths were constructed using thin silicon etalons separated by empty gaps. Due to the large difference between the index of refraction of silicon (3.384) and vacuum (1), calculations indicate that only three periods are required to produce 99.9% reflectivity. The mirror was assembled from high purity silicon wafers, with resistivity over 4000 ohm-cm to reduce free carrier absorption. Wafers were double side polished with faces parallel within 10 arc seconds. The multi-layer mirror was demonstrated as a cavity mirror for the far-infrared p-Ge laser.
Microwave Technologies is developing a revolutionary miniature terahertz traveling-wave microtube (TTM) that will provide sub-millimeter wave radiation for many civilian and military applications. This new concept uses a dielectric microtube in conjunction with a microscopic electron beam. The electron beam is produced by a single micron-sized emitter, which lies underneath the microtube to produce high-power terahertz electromagnetic radiation. The TTM should be easily fabricated using state-of-the-art solid-state technology, and will be a pioneering step towards combining vacuum tube technology with solid-state microfabrication technology. Some of the applications for these exciting new devices include terahertz high-resolution radar, THz chemical and biological sensing, commercial THz line-of-sight networking and ultrahigh-speed computers. A key application of the device under development will be as a miniature terahertz source for biological and chemical spectroscopy. We present detailed numerical and computational analysis of this concept. We also present initial experimental testing of a dielectric microtube designed to operate at 0.1 THz. Once successfully developed, TTMs will be the basis for a new generation of high power terahertz sources capable of producing ultrahigh frequency radiation with high efficiency in an amazingly compact and lightweight package.
Over the past decade the experimental technique of THz time domain spectroscopy (THz-TDS) has proved to be a versatile method for investigating a wide range of phenomena in the THz or far infrared spectral region from 100 GHz to 5 THz. THz-TDS has wide potential for sensing and imaging. The experimental technique is described along with recent results on THz beam propagation for long base-line THz measurements. THz imaging has been demonstrated using both quasi-optical and synthetic aperture approaches, results are presented including images of scatterers as well as non-destructive evaluation of ceramics. Two potential sensing applications of THz-TDS are discussed, thin film characterization and use of waveguides for sensing.
Monte Carlo simulation of carrier dynamics and far-infrared absorption was performed to test the importance of electron-electron interaction in selectively doped multi-layer p-Ge laser at high doping concentration. The laser design exploits the known widely tunable mechanism of THz amplification on inter-sub-band transitions in p-Ge, but with spatial separation of carrier accumulation and relaxation regions, which allows remarkable enhancement of the gain. The structure consists of doped layers separated by 200 - 500 nm of pure-Ge. Vertical electric field (~ 1 - 2 kV/cm) and perpendicular magnetic field (~ 1 T) provide inversion population on direct intersubband light- to heavy-hole transitions. Heavy holes are found to transit the undoped layers quickly and to congregate mainly around the doped layers. Light holes, due to tighter magnetic confinement, are preferably accumulated within the undoped layers, whose reduced ionized impurity scattering rates allow higher total carrier concentrations, and therefore higher gain, in comparison to bulk p-Ge lasers. Preliminary results of the calculations show a possibility of laser operation at liquid nitrogen temperatures. Device design and diagnostics of CVD grown structure are presented. Combination of total internal reflection and quasi-optical cavity design provides high laser cavity Q.
A far-infrared p-type germanium laser with active crystal prepared from ultra pure single-crystal Ge by neutron transmutation doping (NTD) is demonstrated. Calculations show that the high uniformity of Ga acceptor distribution achieved by NTD significantly improves average gain. The negative factor of stronger ionized impurity scattering due to high compensation in NTD Ge is shown to be unremarkable for the gain at moderate doping concentrations sufficient for laser operation. Experimentally, this first NTD laser is found to have lower current-density lasing threshold than the best of a number of melt-doped laser crystals studied for comparison.
Terahertz imaging has the potential to reveal concealed explosives; metallic and non-metallic weapons (such as ceramic, plastic or composite guns and knives); flammables; biological agents; chemical weapons and other threats hidden in packages or on personnel. Because terahertz imaging employs safe non-ionizing radiation that penetrates clothing, people may be routinely scanned as well as packages. Time domain terahertz imaging can be employed in reflection mode to image beneath clothing with sub millimeter resolution. Fiber optic coupled terahertz transmitter and generator arrays can be constructed to more quickly objects such as shoes, or larger portions of the body. The application of commercially available time domain terahertz spectroscopy equipment to imaging through clothing on simulant personnel is shown to distinguish harmful from innocuous objects.
We review the current status of terahertz technologies, especially terahertz beam generators and detectors, which include those being developed in our laboratory. Of the many promising terahertz technologies, only a few may currently be suitable for military applications in the field, which require a high-power terahertz output and a high sensitivity in detection. In addition to these requirements, terahertz imaging for military applications demands focal-plane-arrayed detectors. Presently we are developing a terahertz source based on a modified difference-frequency technique, in which we employ an optical parametric method. We used a Nd:YAG Q-switched laser as a pump source, and LiNbO3 and GaSe crystals as the optical parametric medium. The phase matching condition is achieved by means of our new electro-optical tuning technique. Our preliminary experiments indicate that the new tuning technique enhances the terahertz-beam output power. Although the output power is currently unstable, our source can produce terahertz peak power as high as a few watts. For terahertz detection we use a Si-bolometer or an electro-optic (EO) detector. In addition to these sensors we are presently developing a new detector based on a quantum dot structure. The sensitivity of the quantum dot detector is expected to be about 10-21 W/(Hz)1/2 in terms of the noise equivalent power (NEP); this is orders of magnitude better than the sensitivity of our bolometer (10-13 W/(Hz)1/2) at 4.2 K or our EO detector (10-12 W/(Hz)1/2) at room temperature.
We discuss advances in the optoelectronic generation and detection of coherent terahertz radiation. The demonstrations utilize poled polymeric media, which exhibit characteristics that are extremely well suited for broadband THz applications. Using a 35 mm thick poled polymer film, we demonstrate electro-optic detection of freely propagating ultrashort baseband electromagnetic pulses with spectral sensitivity that extends from the far-infrared (l ~ 100 μm) to ~33 THz (l = 9 μm). Over a band of nearly 20 THz, a relatively flat frequency response is observed. We compare the performance of the poled polymer with that of established materials. We also present a novel waveguide geometry that allows for the phase-matched generation of broadband THz radiation in a polymer-based parallel plate metal waveguide via optical rectification. Both the optical pump beam and the generated THz radiation propagate in the fundamental mode of the waveguide. This allows for non-critical phase matching over a broad range of THz frequencies. We demonstrate guided wave interaction lengths of up to 3 mm.
Passive millimeter wave (PMMW) imaging systems have been developed at Ka-band using imaging elements with Fermi tapered slot antennas (TSA) with corrugation. The imaging elements were installed in a focal-plane imaging system and a Fresnel (close)-distance imaging system to obtain medical and bio-object images.
We demonstrate the use of terahertz time domain spectroscopy for determination of ligand binding for biomolecules. Vibrational modes associated with tertiary structure conformational motions lay in the THz frequency range. We examine the THz dielectric response for hen egg white lysozyme (HEWL): free and bound with tri-N-acetyl-D-glucosamine. Transmission measurements on thin films show that there is a small change in the real part of the refractive index as a function of binding and a sizable decrease in the absorbance. A phenomenological model is used to determine the source of the absorbance change. A change in the vibrational mode density of states and net dipole moment changes will necessarily happen for all biomolecule-ligand binding, thus THz dielectric measurements may provide an universally applicable method to determine probe-target binding for biosensor applications.