Anomalous reflection of THz pulse, containing a few cycles, from absorbing layer: influence of absolute phase of the pulse on the medium response

Abstract

We analyze an interaction of THz pulse containing a few cycles with absorbing or transparent layer. The process under consideration is described by 1D Maxwell’s equations. This analysis is very important for problem of the detection and identification of substance because at the interaction of such THz pulse with layer, the pulse spectrum can essentially change in dependence of a layer thickness. As consequence, excitation of different energy levels occurs in spite of the spectrum unchanging for the incident pulse. Therefore, the medium response can change essentially due to changing of emission frequencies. We found out the non-monotonic dependence of a reflection coefficient from both absorption coefficient, and layer thickness, and absolute phase of the pulse. For the detection and identification of substances, this dependence leads to additional requirements with respect to the incident THz pulse spectrum. For absorbing layer, we see non-monotonic dependence of absorption energy from the layer thickness and its reflection coefficient. This dependence has maximum with changing of the absorption coefficient because a reflection of laser energy is a function of both dielectric permittivity of a medium and its absorption coefficient. Amplitude of reflected THz pulse depends in strong way from the absolute phase of incident THz pulse if the pulse duration is sufficient small.