P. C. Upadhya

Far-Infrared Spectroscopic Characterization of Explosives for Security Applications Using Broadband Terahertz Time-Domain Spectroscopy

Broadband terahertz time-domain spectroscopy (THz-TDS) has been used to measure the far-infrared (FIR) vibrational spectra of several commonly used pure explosives, including 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), 1,3-dinitrato-2,2-bis(nitratomethyl)-propane (PETN), and two types of plastic explosive, SEMTEX and SX2. A number of distinct absorption peaks, originating from FIR-active vibrational modes of these polycrystalline energetic materials, were observed in the frequency range 0.3–7.5 THz (10–250 cm−1). In addition, the temperature-dependent FIR vibrational spectra of PETN were measured between 4 K and 296 K with several well-resolved absorption peaks observed across this temperature range. We find that as the temperature is reduced, the observed absorption peaks resolve into narrower features and shift towards higher frequencies. The temperature dependence of the spectra is explained in terms of the anharmonicity of the vibrational potentials of crystalline compounds, and an empirical fit is given to describe the peak shift with temperature.

Excitation-density-dependent generation of broadband terahertz radiation in an asymmetrically excited photoconductive antenna

The generation of terahertz (THz) transients in photoconductive emitters has been studied by varying the spatial extent and density of the optically excited photocarriers in asymmetrically excited, biased low-temperature-grown GaAs antenna structures. We find a pronounced dependence of the THz pulse intensity and broadband (>6.0 THz) spectral distribution on the pump excitation density and simulate this with a three-dimensional carrier dynamics model. We attribute the observed variation in THz emission to changes in the strength of the screening field.

Broadband terahertz time-domain and Raman spectroscopy of explosives

Broadband terahertz time-domain spectroscopy (THz-TDS) has been shown to be a valuable technique for the detection and analysis of explosives. In this paper, we present recent work on the use of this technique to analyse two pure explosives, 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and 1,3-dinitrato-2,2-bis(nitratomethyl)propane (PETN) and three plastic explosives (Semtex, SX2 and Metabel). For each explosive, a clear and unique spectral absorption fingerprint is found, which can be attributed to oscillations of the molecular lattice. Temperature dependent terahertz absorption measurements of RDX show that the observed vibrational modes are anharmonic in nature. Terahertz spectra are compared with Raman spectra, and the similarities and differences between the spectra produced by the two techniques are discussed. A molecular mechanics calculation approach is used to calculate vibrational intensities and frequencies, and these results are compared with experimental results at both room temperature and 4 K.

On-chip photoconductive excitation and detection of pulsed terahertz radiation at cryogenic temperatures

We report on measurements of the excitation, propagation, and detection of picosecond duration electrical pulses at cryogenic (∼4K) temperatures in a microstrip circuit. A reduction is observed in the measured excitation and propagating pulse widths at low temperatures, compared with room temperature. The results indicate both that the electrical properties of low-temperature-grown GaAs make it suitable for photoconductive excitation and detection in cryogenic optoelectric circuits and that an organic polymer is a good transmissive medium for terahertz (THZ) frequency range excitations at low temperatures. This work will prove an invaluable starting point for future guided wave terahertz experiments at cryogenic temperatures.

Terahertz time-domain spectroscopy of crushed wheat grain

The terahertz time-domain spectroscopy (THz-TDS) is used to measure absorption spectra in crushed wheat grain over the frequency range of 0.1-2.0 THz. A number of well resolved absorption peaks are observed for dry, 12%, 14% and 18% moisture content levels. Subtracting the transmission signal spectra of the dry flour from that of the wetted flour provides a calibrated result which relates attenuation to moisture content level.

Complementary spectroscopic studies of materials of security interest

We demonstrate that, through coherent measurement of the transmitted terahertz frequency electric fields, broadband (0.3 - 8 THz) time-domain spectroscopy can be used to measure far-infrared vibrational modes of a range of drugs-of-abuse and high explosives that are of interest to the forensic and security services. Our results indicate that absorption features in these materials are highly sensitive to the structural and spatial arrangement of the molecules. Terahertz frequency spectra are also compared with high-resolution low-frequency Raman spectra to assist in understanding the low-frequency inter- and intra-molecular vibrational modes of the molecules.

Analysis of drugs-of-abuse and explosives using terahertz time-domain and Raman spectroscopy

We demonstrate that, through coherent measurement of the transmitted terahertz electric fields, broadband (0.3-8THz) time-domain spectroscopy can be used to measure far-infrared vibrational modes of a range of illegal drugs and high explosives that are of interest to the forensic and security services. Our results show that these absorption features are highly sensitive to the structural and spatial arrangement of the molecules. Terahertz frequency spectra are also compared with high-resolution low-frequency Raman spectra to assist in understanding the low frequency inter- and intra-molecular vibrational modes of the molecules.

Monolithic integration of low-temperature-grown GaAs with a two-dimensional electron gas

A scheme is presented for the monolithic integration of low-temperature- grown gallium arsenide (LT-GaAs) with a two-dimensional electron gas. Pulsed terahertz (THz) emission (with a 3 THz bandwidth) is demonstrated from a LT-GaAs layer grown epitaxially above a GaAs–AlGaAs heterojunction containing a two-dimensional electron gas, using photoconductive antennae patterned on the LT-GaAs, and excited with 800 nm, 12 fs laser pulses. Electrical transport measurements of the two-dimensional electron gas at cryogenic temperatures, and in high magnetic fields, reveal quantum Hall behaviour. The THz emission characteristics of the LT-GaAs, and the transport properties of the two-dimensional electron gas demonstrate that this integrated approach compromises neither the quality of the LT-GaAs nor the two-dimensional electron gas, and indicate the potential of this monolithic integration for sub-picosecond electronic measurements of mesoscopic systems.

Time-domain terahertz spectroscopy and applications on drugs and explosives

Many materials of interest to the forensic and security services, such as explosives, drugs and biological agents, exhibit characteristic spectral features in the terahertz (THz) frequency range. These spectral features originate from inter-molecular interactions, involving collective motions of molecules. Broadband THz time-domain spectroscopy (THz-TDS) system have been used to analyze a number of drugs-of-abuse and explosives that are of interest to the forensic and security services. These samples ranged from crystalline powders, pressed into pellets, to thin sheets of plastic explosives, and all being measured in transmission geometry in the frequency range 0.1 - 8 THz. To well understand the nature of the observed spectral features and the effects of thermal broadening on these far-infrared signatures, temperature-dependent THz-TDS measurements have also been performed at temperatures as low as 4 K, especially for two types of cocaine. Well-resolved low-frequency absorption peaks were observed in the frequency range 0.1 - 3 THz with high resolution. Some of absorption peaks were found clearly to become more intense and shift to higher frequencies as the temperature was reduced. The results confirm that the low-frequency collective modes are highly sensitive to the structural and spatial arrangement of molecules. Furthermore, a number of common postal packaging materials made from paper, cardboard, even several types of plastic, have been tested with drug sample to assess the ability of THz-TDS in a hostile detection environment.

Terahertz time domain spectroscopy - present and future modalities

We overview and compare free-space and on-chip terahertz time-domain spectroscopy systems. Example spectroscopic applications are given for both types of system: vibrational spectroscopy of crystalline compounds for free-space systems, and assay of dielectric films for on-chip systems.