Terahertz magneto-photocurrents in the topological insulator Bi$_2$Se$_3$ probe its topological surface states

We study ultrafast magneto-photocurrents in a three-dimensional topological insulator. For this purpose, we excite (In$_r$Bi$_{1-r}$)$_2$Se$_3$ thin films with a femtosecond laser pulse in the presence of an external magnetic field $B_{\text{ext}}$ up to 0.3 T parallel to the film plane. The resulting in-plane photocurrent is measured by detecting the emitted terahertz (THz) electromagnetic pulse. It scales linearly with $B_{\text{ext}}$ and is perpendicular to $B_{\text{ext}}$. Strikingly, for $r\ge$4%, we observe an abrupt photocurrent reduction, which is strongly correlated with the Indium-induced quenching of the topological surface states. The rise time, decay time and amplitude of the THz magneto-photocurrent can consistently be explained by a scenario in which optically excited spin-polarized electrons propagate toward the film surface where the accumulated spin is converted into an in-plane charge current due to spin-velocity locking. Our results are highly relevant for contact-free probing of spin-charge conversion in systems with paramagnetic rather than spontaneous magnetic order.