Thickness-dependent terahertz emission from Bi2S3 films under excitation below and above the band gap

Two-dimensional (2D) materials demonstrate fascinating thickness-dependent optical properties due to their van der Waals interaction and quantum confinement in the linear optical regime. However, the thickness-dependent nonlinear optical response is more complicated as virtual carriers (dipoles) and real carriers can be generated by below- and above-band-gap excitation, which calls for a more comprehensive understanding of these carriers in 2D materials. Herein, a direct band gap ${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ is utilized to investigate the thickness-dependent nonlinear optical response by a terahertz (THz) emission spectroscopy. The results suggest that the THz emission intensity decreases with the increase of band gap energy under an 800-nm femtosecond laser excitation, which can be described by an empirical exponential equation. Under below-band-gap excitation, the optical rectification effect induced by instantaneous polarization (virtual carriers) dominates the THz emission mechanism in thin ${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ films. In contrast, under above-band-gap excitation, real carriers are controlled by both resonant optical rectification and surface depletion electric field effects, resulting in shift and drift currents in thick ${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ films. The contribution ratio of the shift and drift currents is $\ensuremath{\sim}1:1$ at ${45}^{\ensuremath{\circ}}$ oblique incidence. This competition between shift and drift currents results in the elliptically polarized THz wave generation, and the major axis orientation and ellipticity of the ellipse could be manipulated by changing the polarization of the pump light. In this paper, we imply the potential for designing tunable on-chip THz sources and nonlinear optoelectronic devices based on thickness-dependent 2D materials.