Nonlinear Optical Response on the Surface of Semiconductor SnS2 Probed by Terahertz Emission Spectroscopy

Surface nonlinear optical response of layered tin disulfide (SnS2) is investigated by terahertz (THz) emission spectroscopy using a femtosecond photoexcitation at 400 nm. Under above-bandgap excitation, we distinguish the surface optical rectification, surface electric field (SEF)-induced optical rectification, and SEF-induced drift photocurrent. The pump fluence dependence of the THz emission demonstrates a saturation effect due to the electrostatic screening by high photocarrier density on the surface. Theoretical calculations suggest that the SEF-induced optical rectification is negligible. Under the normal incident pump, the surface optical rectification makes the main contribution to the detected THz radiation. However, the drift photocurrent and surface nonlinear polarization contribute competitively to the detected THz radiation at the oblique incidence. The total contribution of drift photocurrent to the detected THz radiation can be achieved as high as ∼60% at 40° incidence calculated from the azimuthal angle dependence. The results provide an understanding of the nonlinear optical effect on the surface of layered semiconductors and afford a versatile spectroscopic tool for surface nonlinear optical properties.