Near-field terahertz emission by transient photocurrents: Monte-Carlo simulation and comparison to experiment

Monte Carlo simulations of photoexcited carrier dynamics combined with time domain electromagnetic modeling of the radiated field reveal different roles of the photocurrent vector components in the process of THz pulse generation upon ultrashort optical excitation of semiconductor surfaces. We find that only low frequency components (below 0.3 THz) are generated mainly by the initial transient current normal to the surface. Higher frequencies (0.6-1.0 THz) have a stronger contribution from the current transients in the plane of the semiconductor surface, whereas photo-excited plasma oscillations in the direction normal to the surface are found significant above 1.5 THz. The simulation results are verified using experimental near-field mapping.