Terahertz emission and optical second harmonic generation from Si surfaces

We study polarity reversal in terahertz pulses generated from p-doped Si(001) surfaces with different doping levels. The azimuthal dependence of both terahertz emission and second-harmonic generation caused by the bulk electric quadrupole and magnetic dipole process is measured to determine the contributions of optical rectification and the surface depletion field direction, respectively. By comparing the penetration depth of the excitation laser and the depletion field strength on Si surfaces, we show that competition between the photo-Dember effect and surface field accelerations causes the polarity reversal.