Detection and identification of explosives hidden under barrier materials - what are the THz-technology challenges?

We describe experiments where different explosives were hidden under common barrier materials, and THz radiation was used to detect and identify these explosives. Our THz system, a time-domain spectroscopy (TDS) system, is based on a femtosecond laser whose radiation is converted into THz radiation by a low-temperature grown GaAs photoconductive switch. A similar switch detects the reflected signal. The advantage of using a TDS system is that pulses reflected from the barrier and the actual explosive, arrive at different instances at the detector. This simplifies the separation of the barrier signature from the explosive signature, compared to a frequency domain system. However, partial temporal overlap between the two pulses makes it challenging to completely separate the spectral characteristics of the explosive from the characteristics of the barrier. Also, in addition to attenuating the THz-pulses, transmission through barrier materials may add spectral features to the reflected signal, hampering recognition of the explosive. On top of that, the explosive may have a rough surface, which reduces the strength of the reflected signal. In this contribution we shall address these issues and discuss strategies that may be used to face these challenges.