Tunable polarized terahertz wave generation induced by spontaneous polarization-dependent ultrafast shift current from vertically grown ferroelectric SnS

Abstract

Ferroelectric group-IV monochalcogenides $MX$ ($M=\mathrm{Sn}/\mathrm{Ge}$ and $X=\mathrm{S}/\mathrm{Se}$) are of particular importance for the new-generation photovoltaic devices due to their special in-plane spontaneous polarization resulting in a bulk photovoltaic effect. However, the spontaneous polarization and its related dynamical photoresponse under an ultrafast laser excitation are rarely studied via a noncontact all-optical method. Herein, the vertically grown SnS nanosheets have been synthesized by a physical vapor deposition (PVD) method and the corresponding spontaneous polarization is characterized via the terahertz (THz) emission spectroscopy. The primary THz emission mechanism is ascribed to the shift current, which is proved by the THz wave dependences on the pump light polarization angle and pump fluence. Interestingly, the shift current depends on the spontaneous polarization along the SnS nanosheet grown direction, according to the reversed polarity of the THz wave under opposite directional excitations. Furthermore, the amplitude and phase of the THz wave are modulated by the pump light polarization angle, resulting in the elliptical THz wave emission with tunable ellipticity and orientation angle. Our study has shed light on the shift current of ferroelectrics and controllable polarized THz wave generation, which provides application prospects for ferroelectric-based photovoltaics, nonlinear photonics, and THz polarized devices.