Competition between linear and nonlinear processes during generation of pulsed terahertz radiation in a ZnTe crystal

The generation of terahertz (THz) pulses by the optical rectification of femtosecond laser pulses in a ZnTe crystal is studied. A substantial decrease in the THz radiation power was observed upon tight focusing of laser radiation into the crystal. It is shown that the consideration of competing two-photon absorption and second-harmonic generation processes proceeding simultaneously with optical rectification cannot explain this effect even qualitatively. It is assumed that the observed decrease in the THz radiation power is caused by a decrease in the size of a source of nonlinear polarisation. The conditions are found for the most efficient generation of THz radiation in the ZnTe crystal.