Controlling helicity-dependent photocurrent in polycrystalline Sb$_2$Te$_2$Se topological insulator thin films at ambient temperature through wave-vector of and photothermal gradient due to polarized light

Abstract: Optical control of helicity-dependent photocurrent in topological insulator Sb$_2$Te$_2$Se has been studied at room temperature on dominantly c-axis oriented granular polycrystalline samples grown by pulsed laser deposition technique. Strong spin-orbit coupling and spin-momentum locking make this system unique for their applications. We observed that photocurrent can be controlled by exciting the sample with different circular and linear polarized light, yielding a polarization-dependent current density which can be fitted very well with a theoretical model. Magnitude of the photocurrent is higher even at room temperature, compared to previous reports on other single-crystal topological insulators. Comparison with the theoretical model suggests that photocurrent has different contributions. Study of dependence of photocurrent on the angle of incidence (wave-vector) of the excitation laser beam with respect to the surface normal of the sample helps to identify origins of different terms contributing to the observed photocurrent. Incidence-angle driven helicity switching, which is a very simple and effective technique to control the directional photocurrent, has also been observed in this study. This photocurrent can also be controlled with the help of photothermal gradient generated by the excitation light beam. Enhancement and inversion of this photocurrent in presence of photothermal gradient for light incident on two opposite edges of the sample occur due to selective spin state excitation with two opposite (left and right) circularly polarized light in presence of the unique spin-momentum locked surface states. These observations renders this polycrystalline material to be more important in polarization-dependent photodetection applications as well as for spin-optoelectronics under ambient conditions.