The absorption spectrum of the explosive 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) has been measured using a conventional Fourier transform infrared spectroscopy and by terahertz pulsed spectroscopy. Seven absorption features in the spectral range of 5–120cm−1 have been observed and identified as the fingerprint of RDX. Furthermore, the spatial distribution of individual chemical substances including RDX, has been mapped out using reflection terahertz spectroscopic imaging in combination with component spatial pattern analysis. This is the terahertz spectroscopy and chemical mapping of explosives obtained using reflection terahertz measurement, and represents a significant advance toward developing a terahertz pulsed imaging system for security screening of explosives.

Using an analytical theory, experimental terahertz time-domain spectroscopy data, and numerical evidence, we demonstrate that the frequency dependence of the absorption coupling coefficient between far-infrared photons and atomic vibrations in disordered materials has the universal functional form, C(omega)=A+Bomega(2), where the material-specific constants A and B are related to the distributions of fluctuating charges obeying global and local charge neutrality, respectively.

A prototype terahertz standoff detection system is shown to be able to sense explosives at a distance of one meter, through several layers of clothing, employing safe-to-use non-ionizing radiation.ABSTRACT | Terahertz imaging and spectroscopy has been shown to have the potential to use very low levels of nonionizing radiation to detect and identify objects hidden under clothing. In this paper we discuss some of the important factors involved in addressing practical systems in the security industry, and describe our recent work on the development of a prototype terahertz standoff detection system. Using this system we demonstrate the spectroscopic detection of concealed explosives at a standoff distance of 1 m, both real time, in reflection, and under normal atmospheric conditions.

An imaging system in reflection geometry based on a multimode 2.9 THz quantum cascade laser as radiation source is reported. The beating between neighbouring longitudinal modes is detected using a room temperature point-contact Schottky diode as mixing element. We show that the technique can, in principle, give a dynamic range of 60 dB with a time constant of ~ 10 mus.

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