Propagation of laser pulse with a few cycle duration in multi-level media

1D propagation of a THz Gaussian pulse, containing a few cycles, in multi-level media is studied using computer simulation. Electromagnetic field evolution is governed by Maxwell´s equations whereas a medium response is described in the frame-work of the density matrix formalism. For the pulse with a sufficiently broad spectrum we demonstrate the appearance of a cascade mechanism for molecule energy level excitation. As a result, the spectrum of the THz pulse, transmitted through a medium layer, is broadened essentially in comparison with the incident pulse spectrum. We also take into account the incident pulse absolute phase influence on the pulse spectrum broadening.