Dynamics of photoinduced field screening; THz-pulse and second harmonic generation from semiconductor surface

The equation to describe the evolution of a space charge electric field in a semiconductor after an ultra-short laser pulse excitation is extended to be valid for both drift-diffusion and collisionless transport regimes. Numerical calculations of this equation performed for a semiconductor surface depletion region show that the character of the surface electric field screening varies from monotonic to oscillatory with increasing photoexcitation level. It is found that THz-pulse emission from the excited surface depletion layer is determined by the second time derivative of the surface potential, and its waveform shows bipolar or oscillatory behaviour depending on whether drift-diffusion or collisionless transport regime is realized. The depletion field induced second harmonic (SH) contribution is analysed.