Role of the pulse duration at measurements of spectral signatures of substances in THz range of frequencies

Abstract

THz TDS is one of tools, which can be effective for the security problem solution. As a rule, the detection and identification of substance is based on the spectral fingerprints: absorption frequencies of substance. To detect the substance, it is necessary to compare its absorption frequencies with those from database which contains the absorption frequencies of various substance. Therefore, a role of this data is one of the keys for the problem solution. As follows from analysis of papers, the measurements of the absorption frequencies were provided by using broadband THz pulse. Due to essential non-stationary response of a medium as well as broadband spectrum of the THz pulse, in the spectrum of pulse, transmitted through a substance, many additional frequencies appear. Among them maybe appear “false” absorption frequencies, appearance of which is caused by frequency conversion processes, as well as the frequencies, corresponding to a substance emission because of a high energy levels excitation due to the cascade mechanism. Both these mechanisms of the spectrum transformation strongly depend on the incident pulse duration. Another additional factor resulting in observing of the false absorption frequencies is disordered (or ordered) packing for the substance. In THz range of frequencies, the most part of the packing is a disordered photonic structure and developing of the strategy to avoid its influence on the efficiency of detection and identification of substance is a key problem. We discuss once more approach for solving this problem in this paper.