Nonlinear Transfer of Intense Few Cycle Terahertz Pulse Through Opaque Semiconductors

Extremely intense few-cycle terahertz pulses exhibit complex nonlinear behavior under interaction with n-doped opaque Si having low field transmission of similar to 0.02%. Transmission of a 700-fs pulse with central frequency similar to 1.5 THz through the Si sample increases up to similar to 8% for the external field strength of 5 MV/cm and then drops twofold at similar to 20 MV/cm. Electro-optical sampling measurements revealed formation of a single-cycle terahertz pulse at this field due to generation of a thin ionized layer by the first intense oscillation of the field. We explained the data obtained from the competition of processes of collision electron-phonon rate fast increase and saturation, as well as ionization under action of an intense THz field.