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Substance identification by pulsed THz spectroscopy in the presence of disordered structure

We discuss an effective method for the detection and identification of a substance, covered by a disordered structure, using the pulsed THz signal. The insufficiency of the standard THz-TDS method, based on the comparison of substance absorption spectra from database with the absorption spectrum of the substance under investigation, is demonstrated. To explain a physical mechanism of false absorption lines appearance in the signal we make a computer simulation on the base of 1D Maxwell's equations and density matrix formalism. For the detection and identification of substance we propose to utilize substance emission at high frequencies corresponding to the high energy levels excitation under the THz pulse action due to the cascade mechanism. In the case of a non-resonant medium we also discuss second harmonic generation and a possibility of its application to the substance detection and identification. The proposed method of the substance detection and identification is based on time-dependent integral correlation criteria calculated with the help of the spectral dynamics of medium response. A new type of the integral correlation criterion, which is less dependent on spectral characteristics of the noisy signal under investigation, is used for the identification.