Diffusion and drift in terahertz emission at GaAs surfaces

We study terahertz (THz) emission from GaAs as a function of photon energy and electric field. THz radiation arises from transport of photogenerated charge in an electric field and by hot carrier diffusion (the photo-Dember effect). These mechanisms can be separated by experiments in which either the electric field or the kinetic energy of the carriers is varied. For electric fields E∼4 kV/cm, we find that the electric field controls THz emission for carrier temperatures kBTC⩽0.1 eV, while hot-carrier diffusion dominates for kBTC≈1 eV. Both mechanisms contribute at intermediate fields and carrier temperatures. Our results are consistent with estimates of the relative magnitudes of these two effects.