  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 yet at cryogenic
temperatures. Here, we show that surface currents in Bi2Se3 can be controlled
by circularly polarized light on a picosecond time scale with a fidelity near
unity even at room temperature. We re-veal 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.
