Continuous-wave terahertz (THz) radiation spectroscopy was performed on high explosive materials using a tuneable optical parametric oscillator (OPO). Military grade, solid-phase, explosive substances, such as cyclotetramethylenetetranitramine (HMX), cyclotrimethylenetrinitramine (RDX), pentaerythritol tetranitrate (PETN), and composition-4, were spectrally scanned over the 0.7–1.9？THz frequency range under experimental conditions modeling that of “real-world” security screenings. Spectral peak locations and spectral line broadening effects were quantified using a Lorentz lineshape fit algorithm. The full-width half-maximum (FWHM) parameter computed by the Lorentz fit algorithm was shown to help in the identification of samples with broad and sparse spectral characteristics. A concealed explosives identification scheme was demonstrated through raster scan THz frequency radiation imaging at specific OPO tuning frequencies. 1. Introduction Terahertz (THz) radiation, typically recognized as the 0.3 to 10？THz frequency region (wavelengths of 1？mm to 30？μm), offers promising solutions to many of the challenges facing modern security and surveillance systems. THz radiation can penetrate most common dielectric materials enabling imaging through plastics, clothing, and cardboard. Since metal strongly reflects THz radiation, concealed metallic weapons can be easily identified via their high contrast shapes and contours within a THz radiation scanned image. Unlike X-rays, THz radiation photon energy is low and non-ionizing (1？THz？=？4？meV) so there are no adverse health effects associated with prolonged exposure [1]. The allure of THz radiation technology for security applications also stems from the fact that the collective intermolecular motions and vibrational transitions of materials, such as explosives and illicit drugs, can be probed with THz frequency radiation [2]. Neither millimeter waves nor infrared (IR) waves have access to these molecular fingerprints. It is envisaged that THz radiation spectroscopy can be incorporated into a holistic security system that enables noninvasive identification of substances according to their THz frequency absorption profile while simultaneously imaging packages or persons for concealed weapons. There have been technological advancements in the development of continuous-wave (CW) THz radiation sources and detectors. Optically pumped THz Lasers (OPTL) [3], Backward-Wave Oscillators (BWO) [4], Photomixers [5], and Quantum Cascade Lasers (QCL) [6] are capable of efficiently generating CW THz radiation while detection of the
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