Terahertz Science and Technology for Military and Security Applications

The following sections are included:

• FOREWORD

• CONTENTS

The analytical applications of terahertz (THz) spectroscopy for the characterization of molecular solids have been limited by the lack of information concerning the assignment of observed spectral features to specific internal (intramolecular) and external (intermolecular) atomic motions. Computational methodologies addressing the assignment of spectral data are the enabling technology for moving THz spectroscopy to the forefront of available detection methods for both imaging and spectroscopic applications. Solid-state density functional theory (DFT) studies have been performed on the high explosives cyclotetramethylenetetranitramine (HMX) and pentaerythritol tetranitrate (PETN) in order to address the dependencies of the predictions of solid-state vibrations in the terahertz (3 to 120 cm^-1) region on the choice of basis set and integration grid size, building on previous work that examined this dependency on the choice of density functional. DFT THz simulations reveal that both the choice of basis set and grid size have important influences on the reproduction of spectral features. The sensitivity to basis set choice is most pronounced in the calculation of vibrational intensities, where it is found that THz absorption intensities are most accurately reproduced when derived from basis set-sensitive Mulliken atomic charges as opposed to basis set-insensitive atomic charges generated by the Hirshfeld partitioning method.

We apply THz imaging technology to evaluate fire damage to a variety of carbon fiber composite samples. The majority of carbon fiber materials have polarization-dependent reflectivities in the THz frequency range, and we show how the polarization dependence changes versus the burn damage level. Additionally, time domain information acquired through a THz time-domain spectroscopy (TDS) system provides further information with which to characterize the damage. The technology is discussed in terms of non-destructive testing applications to the defense and aerospace industries.

The development of an effective biological (bio) agent detection capability based upon terahertz (THz) frequency absorption spectra will require insight into how the constituent cellular components contribute to the overall THz signature. In this work, the specific contribution of ribonucleic acid (RNA) to THz spectra is analyzed in detail. Previously, it has only been possible to simulate partial fragments of the RNA (or DNA) structures due to the excessive computational demands. For the first time, the molecular structure of the entire transfer RNA (tRNA) molecule of E. coli was simulated and the associated THz signature was derived theoretically. The tRNA that binds amino acid tyrosine (tRNA_tyr) was studied. Here, the molecular structure was optimized using the potential energy minimization and molecular dynamical (MD) simulations. Solvation effects (water molecules) were also included explicitly in the MD simulations. To verify that realistic molecular signatures were simulated, a parallel experimental study of tRNAs of E. Coli was also conducted. Two very similar molecules, valine and tyrosine tRNA were investigated experimentally. Samples were prepared in the form of water solutions with the concentrations in the range 0.01-1 mg/ml. A strong correlation of the measured THz signatures associated with valine tRNA and tyrosine tRNA was observed. These findings are consistent with the structural similarity of the two tRNAs. The calculated THz signature of the tyrosine tRNA of E.coli reproduces many features of our measured spectra, and, therefore, provides valuable new insights into bio-agent detection.

We report the sensing and imaging of explosive related chemical and bio-chemical materials by using terahertz time domain spectroscopy (THz-TDS) at standoff distance. The 0.82 THz absorption peak of RDX is observed at a distance up to 30 m away from the emitter and receiver. Multiple absorption features of RDX, 2,4-DNT and Glutamic Acid are identified by using a large scale 2-D imaging system. These results support the feasibility of using THz-TDS technique in remote sensing and detection of chemical materials.

Motivated by the possibility of identifying and detecting certain biochemical species using Fourier transform infrared spectroscopy (FTIR), we have investigated porcine, bovine, lispro, and human insulins. We have successfully observed and identified all the transition peaks for the four types of insulins in the frequency domains from mid-IR to THz. In the mid-IR region, ten transition peaks have been observed for all four insulins. Although these four insulins are made from amino acids which have either slightly different sequences or slight variations, the ten transition frequencies are virtually indistinguishable. However, for protamine sulfate some of the transition frequencies in the same mid-IR region are either red-shifted or blue-shifted relative to the corresponding ones for the four insulins. Furthermore, the strengths for several peaks of protamine sulfate are significantly reduced whereas the strength for only one transition peak is significantly enhanced, compared with the corresponding ones for the insulins. In the far-IR to THz transition region, the shapes and locations of the two transitions are quite different between the insulin species and protamine sulfate. In the THz region we have observed a linear dependence of the absorption coefficients on frequency for each of the four insulins and protamine sulfate.

We report the first systematic study of broadband THz wave generation with ambient air as the nonlinear media. Generation of pulsed THz waves with mixing the fundamental and the second harmonic beams in air has been previously demonstrated by Cook et al. and Hartmut et al while different groups obtained different results. To verify the proposed mechanism of strong THz wave generation, we measured dependence of generated THz field on the polarization, amplitude and phase of the individually controlled two beams. Our results confirm that four-wave-mixing rectification is the major mechanism this phenomenon, and the amplitude and polarity of generated THz wave can be controlled by the relative phase of the beams. This work is significant by providing the feasibility of THz wave generation and detection with a standoff distance greater than 50 meters.

We demonstrate a large area time domain terahertz (THz) imaging system capable of scanning 1 meter square area in less than 20-100 minutes for several security applications. The detection of concealed explosives; metallic and non-metallic weapons (such as ceramic, plastic or composite guns and knives); and flammables in luggage, packages and personnel has been demonstrated. Transmission mode images of luggage containing threat items are discussed. Reflection mode images of luggage and personnel are discussed. Time domain THz images can be analyzed for 3 dimensional and volumetric information. Time domain THz images have advantages over coherent narrow band imaging methods, with freedom from interference artifacts and with greater ability to discard irrelevant or intervening reflections through time discrimination.

Terahertz (THz) spectra of two TNT-related compounds (4-NT and 2, 6-DNT) are investigated using Fourier transform infrared spectroscopy (FTIR, 1.5-20 THz) and THz time-domain spectroscopy (THz-TDS, 0.2-2.5 THz). Density functional theory (DFT) is applied to calculate THz spectra of these two compounds. Transmission, diffuse reflection, and calculated spectra are in good agreement. The measured THz resonance lines from the transmission and diffuse reflectance spectra are assigned based on the DFT simulation. The observed THz signatures imply that THz spectrum has potential for standoff detection of explosives and related compounds (ERCs) in the THz range.

This paper presents models and experimental measurements that shed light on THz-phonon mediated transport of polarons in biomolecules. Polaron transport in DNA has been considered recently in view of the expected delocalization of charge carriers on a one-dimensional wire as well as the highly charged nature of DNA.^1,2 An understanding of the electrical transport properties and THz-phonon interactions of biomolecules is important in view of DNA's potential applications both as electrically conductive wires and as structures that facilitate the chemically-directed assembly of massively integrated ensembles of nanoscale semiconducting elements into terascale integrated networks. Moreover, understanding these interactions provides information of the THz spectrum of vibrational modes in DNA. A primary focus of this paper is on charge transport in biomolecules using indirect-bandgap colloidal nanocrystals linked with biomolecules.³ Through a combination of theoretical and experimental approaches,^4-7 this paper focuses on understanding the electrical properties and THz-frequency interactions of DNA. Moreover, this paper presents observed charge transport phenomena in DNA and discusses how these measurements are related to carrier scattering from the THz vibrational modes in DNA. Indeed, carrier transport in DNA is analyzed in light of theoretical calculations of the effects of THz-frequency phonon emission by propagating carriers, THz-frequency phonon absorption by propagating and trapped carriers, and effective mass calculations for specific sequences of the DNA bases.^1-7 These studies focus on THz-phonon-mediated processes since an extra carrier on a one-dimensional chain minimizes its energy by forming an extended polaron, and since many biomolecules, including DNA, exhibit THz vibrational spectra.⁸ Accordingly, these calculations focus on THz-phonon-mediated processes. These results are discussed in terms of the role of THz-phonon-mediated charge trapping and detrapping effects near guanine-rich regions of the DNA as well as on the understanding and identification of DNA with specific base sequences that promote charge transport. As in previous studies, optical excitation is used to inject carriers into DNA wires. Moreover, this paper reports on the use of gel electrophoresis to study charge- induced cleavage of DNA and the related transport of charge in DNA. Phonon absorption and emission from polarons in DNA,⁹ is analyzed using parameters from the well-known Su-Schrieffer-Heeger Hamiltonian.

The convergence of terahertz spectroscopy and single molecule experimentation offers significant promise of enhancement in sensitivity and selectivity in molecular recognition, identification and quantitation germane to military and security applications. This paper provides a brief overview of the constraints set by single molecule recognition systems and reports the results of experiments which address fundamental barriers to the integration of large, patterned bio-compatible molecular opto-electronic systems with silicon based microelectronic systems. Central to this thrust is an approach involving sequential epitaxy on surface bound single stranded DNA one-dimensional substrates. The challenge of producing highly structured macromolecular substrates, which are necessary in order to implement molecular nanolithography, has been addressed experimentally by combining “designer” synthetic DNA with biosynthetically derived plasmid components. By design, these one dimensional templates are composed of domains which contain sites which are recognized, and therefore addressable by either complementary DNA sequences and/or selected enzymes. Such design is necessary in order to access the nominal 2 nm linewidth potential resolution of nanolithography on these one-dimensional substrates. The recognition and binding properties of DNA ensure that the lithographic process is intrinsically self-organizing, and therefore self-aligning, a necessity for assembly processes at the requisite resolution. Another requirement of this molecular epitaxy approach is that the substrate must be immobilized. The challenge of robust surface immobilization is being addressed via the production of the equivalent of molecular tube sockets. In this application, multi-valent core-shell fluorescent quantum dots provide a mechanism to prepare surface attachment sites with a pre-determined 1:1 attachment site : substrate (DNA) molecule ratio.

A new approach for the development of nano-sized spectroscopic-based early-warning sensors using molecular electrostatic potentials (MEP) and molecular vibronics (MV) was presented. The use of MEPs allow us to sense and detect specific molecules in elaborated arrays of logical gates which provide the signature of the trapped species and a decision signal of the results of the sensing operation. Molecular vibronics is used to activate/deactivate, control and program the detection process as well as to transmit the information to and from nano-micro interfaces that allow the interaction with microelectronic systems. In order to develop this scenario, it is needed to explain the exact reasons, from an atomistic point of view rather than using phenomenological models the effects of molecules on nanoclusters. We present here a study of silicon-phenyl complexes.

A design and analysis study is presented for a new optically-triggered (OT) interband resonant-tunneling-diode (I-RTD) device that has potential for generating terahertz (THz) frequency oscillations and achieving enhanced output power levels under pulsed operation. The proposed device utilizes novel nanoscale mechanisms to achieve externally driven oscillations that consist of two phases – i.e., an initial transient phase produced by a natural Zener (interband) tunneling process and a second discharging transient phase induced by optical annihilation of stored hole-charge by externally-injected photon flux. The specific focus of this paper will be on an OT-I-RTD oscillator that utilizes In_1−xGa_xAs /GaSb_yAs_1−y hetero-systems and the application of band-engineering to enable triggering by 1.55 μm laser technology. The paper presents performance results for the hybrid circuit design, along with a practical implementation strategy for integrating the optical triggering and an analysis of the heating induced during large signal operation.

A combination of Terahertz (THz) polariton spectroscopy and polariton imaging technique for the application to chemical sensing is presented. We use phonon-polaritons, a coupled oscillation of the lattice vibration and radiation field, as an intense radiation source for THz spectroscopy. The propagation process of the polaritons generated in one of the two LiNbO₃ transducer crystals through the sample sandwiched between the crystals is visualized using a polariton imaging technique. Partially reflected polaritons at the transducer-sample interface and polaritons partially transmitted through the sample are visualized simultaneously in a single frame of an image. The temporal profile of reflected and transmitted phonon-polaritons can be obtained without scanning the delay time between the pump and probe femtosecond laser pulses unlike THz time-domain spectroscopy which requires point-by-point acquisition of the temporal pulse profile using conventional pump-probe scheme. The results suggest possible application of this technique to the chemical sensing with fast acquisition rate. The technique has been successfully applied to the measurement of liquid and solid samples, and simultaneous measurement of multiple samples has also been achieved.

False echoes degrade the operation of radar and imaging antennas. The false returns or clutter arise from antenna sidelobes and raise the threshold of detection in perimeter security systems. Accordingly there is great interest in reducing the sidelobes in present day millimeter wave and future terahertz antennas. Here we describe a new technique to suppress antenna sidelobe returns. The technique exploits our ability to distinguish between the phase of desired signals arriving in the antenna main beam and the phase of undesired clutter signals coming from the sidelobes. We demonstrate that through modulation of phase and taking the Fourier transform of the received signal, we can preferentially suppress the clutter return relative to the desired main beam signal. Suppression of average clutter return of over 25 dB is found.

We report the first experimental results for noise-equivalent power (NEP) and noise-equivalent temperature difference (NETD) of single-crystal ErAs:InAlGaAs, zero-bias rectifier diodes coupled to free space quasi-optically in the THz region. At a frequency of 639 GHz, an optical NEP of 4.0×10⁻¹² W/Hz^1/2 is measured with the rectifier coupled to a quasi-plane-wave coherent source through a single-turn square spiral antenna. With a broadband thermal (hot water) source, an NETD of 120 mK is measured from the same device. Antenna radiation patterns at 100 GHz and 639 GHz are also presented.

ALTAIR Center develops long-wave infrared (LWIR) and terahertz-frequency (THz) lasers operating at room temperature employing intraband luminescence in colloidal semiconductor nanocrystals, in which the optical transition frequencies can be easily tuned to the desired values by an appropriate choice of the semiconductor material and radius of the nanocrystals.

Terahertz radiation, which lies between microwave and infrared, has been shown to have the potential to use very low levels of this non-ionising radiation to detect and identify objects, such as weapons and explosives, hidden under clothing. This paper describes recent work on the development of prototype systems using terahertz to provide new capabilities in people screening. In particular, it explores how multi-spectral terahertz imaging and the use of both specularly reflected and scattered terahertz radiation can enhance the detection of threat objects.

There are occasions when identification of individuals in civil clothes or uniform may be necessary. In such a case, the ability of terahertz radiation to react to different configuration of, for example, plastic objects, such as tablets, etc. may be used to give a warning sign if the approaching subject is foreign, be he or she in uniform (the one that looks like your own) or in civil clothes. The identification gives an advantage of “checking the credentials” without asking for them. This feature may be especially useful when a military detachment is located close to the adversary occupied territory. The idea for such an identification device came from real situations similar to those that have taken place in Iraq, etc. The code for identification is built in the password tablet. The tablet coatings are semi-transparent to the THz radiation and do not scatter it significantly. The THz pulses, incident on the tablet surface, penetrate through the different coating layers. At each interface a portion of the pulse is reflected back to the detector. The amplitude of the reflected radiation is recorded as a function of time. The resulting pattern is compared with the password.

Experimental results of homodyne terahertz interferometric 1-D and 2-D imaging are presented. The reconstructed images of a point source are in a good agreement with theoretical predictions. 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 a metal object behind a barrier. 1-D images of a C-4 sample have been obtained at several terahertz frequencies. Focusing issues of 2-D imaging have been demonstrated. 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.