Proceedings Article | 4 January 2008
Proc. SPIE. 6840, Terahertz Photonics
KEYWORDS: Packaging, Signal to noise ratio, Spectroscopy, Crystals, Thermal effects, Terahertz radiation, Explosives, Terahertz spectroscopy, Temperature metrology, Absorption
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.
