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Topological insulators constitute a new and fascinating class of matter withinsulating bulk yet metallic surfaces that host highly mobile charge carrierswith spin-momentum locking. Remarkably, the direction and magnitude of surfacecurrents can be controlled with tailored light beams, but the underlyingmechanisms are not yet well understood. To directly resolve the "birth" of suchphotocurrents we need to boost the time resolution to the scale of elementaryscattering events ($\sim$ 10 fs). Here, we excite and measure photocurrents inthe three-dimensional model topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$with a time resolution as short as 20 fs by sampling the concomitantly emittedbroadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafastsurface current response is dominated by a charge transfer along the Se-Bibonds. In contrast, photon-helicity-dependent photocurrents are found to haveorders of magnitude smaller magnitude than expected from generation scenariosbased on asymmetric depopulation of the Dirac cone. Our findings are also ofdirect relevance for optoelectronic devices based on topological-insulatorsurface currents.

Ultrafast photocurrents at the surface of the three-dimensional topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$