Conversion of short optical pulses to terahertz radiation in a nonlinear medium : experiment and theory.

We report on the frequency and time domain analysis of electromagnetic terahertz radiation generated by nonlinear conversion of short optical pulses crossing boundaries of nonlinear material. Analysis and comparison with experiment have unequivocally established that the nature of collinear radiation at terahertz frequencies relates to the phenomenon of transition radiation produced by the instantaneous creation (at the input interface) and extinction (at the output interface) of the moving polarization charge formed by the nonlinear coupling of the pump electromagnetic fields. The mechanism is analogous to the phenomena of transition radiation of moving free charges, in particular, to the radiation mechanism discussed in the Tamm problem