Ultrafast helicity control of surface currents in topological insulators with near-unity fidelity

In recent years, a class of solid-state materials, called three-dimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spin-polarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions. Here we show that surface currents in Bi 2 Se 3 can be controlled by circularly polarized light on a picosecond timescale with a fidelity near unity even at room temperature. We reveal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators. Bulk contributions to transport measurements often inhibit the study of the surface states of topological insulators. Here, Kastl et al . demonstrate high-fidelity helicity-dependent photocurrents in the surface states of Bi 2 Se 3 , controlled via circularly polarized light with a picosecond time-resolution.