Terahertz surface emission of d -band electrons from a layered tungsten disulfide crystal by the surface field

Terahertz (THz) time-domain emission spectroscopy in both transmission and reflection configurations has been employed to understand the THz radiation property and surface properties of tungsten disulfide ($\mathrm{W}{\mathrm{S}}_{2}$). We observed only one polarization of THz radiation under different polarization of pump beam and a saturation effect with the increasing of pump power. The results are different from that of $\mathrm{Mo}{\mathrm{S}}_{2}$ based on optical rectification in spite of similar physical and optoelectronic properties of them. The nonlinear optical coefficient calculation based on first-principle method combined with the azimuthal angle dependence of THz radiation implies that THz radiation is insensitive to the azimuthal angle in $\mathrm{W}{\mathrm{S}}_{2}$. From the pump polarization angle dependence of THz radiation, we find that the contribution due to the nonlinear effect is only 12% approximately. All these suggest the main THz mechanism from $\mathrm{W}{\mathrm{S}}_{2}$ is due to the surface depletion field induced by the surface states. We also analyzed the surface field features of $\mathrm{W}{\mathrm{S}}_{2}$ with the maximum surface depletion field of approximate $1.2\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{cm}$. Fresnel law combined with the dipole radiation model is also used to analyze the angular dependence of THz radiation. The results can not only afford a fundamental THz radiation property of layered materials, but also promote the development of THz devices based on layered materials.