On the Possibility of Identifying Substances by Remote Active THz Spectroscopy

This paper investigates the limit of detection of the spectral fingerprints of a material to be identified by terahertz (THz) spectroscopy, when the material is bare or concealed by clothes. We propose a theoretical study to determine the minimum observable peak amplitude versus the refractive index and the coefficient of absorption of the sample material. We discuss the case of detecting the absorption peaks from the amplitude or phase of the transmitted or reflected spectra recorded using a THz time-domain spectroscopy system. We experimentally validate our analysis by studying a pellet sample of glutamic acid, bare or hidden under a layer of various fabrics, both in transmission and reflection configuration. This allows us to investigate the influence of various radiometric responses induced by fabrics when spectra are recorded in realistic conditions. In the reflection scheme, interference effects make the identification difficult, if not impossible.