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_{ext} up to 0.3  T parallel to the film plane. The resulting in-plane photocurrent is measured by detecting the emitted terahertz electromagnetic pulse. It is proportional and perpendicular to B_{ext}. Strikingly, for r≥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 terahertz magneto-photocurrent can consistently be explained by the following scenario: 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 without spontaneous magnetic order and without having to add invasive spin-source layers.