Circular photon drag effect in a Bi$_{2}$Se$_{3}$ film probed by terahertz emission spectroscopy

In these days, an optical pumping of topological insulators has been extensively investigated as a route to control the spin-polarized charge current for the spintronic applications. In this work, we address this issue of the spin-polarized photocurrent generation by using terahertz (THz) emission technique. We monitored the THz electric fields emitted from a Bi$_{2}$Se$_{3}$ film with varying a polarization of incident light and sample azimuth, and observed photon-helicity-dependent THz emission responses which exhibit a clear three-fold periodicity in its sample azimuth dependence. Based on a symmetry analysis related to photocurrent generation processes, we demonstrate that the observed circular anisotropy in the photocurrent originates from the circular photon drag effect, namely, linear and angular momentum transfer from photons to photocarriers.