We carry out computer simulation of a few-cycle electromagnetic pulse interaction with a substance exhibiting nonlinear non-instantaneous response. An influence of the pulse duration and polarization relaxation rate on the pulse interaction with an uncovered medium and with a medium, which is covered by a linear disordered structure, is investigated. We demonstrate that the spectra of reflected or transmitted pulses may comprise multiple additional spectral lines, caused by various transitions between energy levels of molecules. The spectral intensity of these energy level transitions is affected by the pulse duration and the effective time of interaction which depends on polarization relaxation rate as well as the medium layer thickness.
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Vyacheslav A. Trofimov, Dmitry Yu. Zagursky, Irina G. Zakharova, "Influence of disordered cover on cascade mechanism of medium response spectrum broadening at THz-TDS of substance," Proc. SPIE 9934, Terahertz Emitters, Receivers, and Applications VII, 993409 (26 September 2016);