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

TL;DR

This study measures ultrafast photocurrents on the surface of the topological insulator Bi2Se3 with 20 fs resolution, revealing charge transfer mechanisms and challenging previous assumptions about photon-helicity effects.

Contribution

It provides the first direct measurement of ultrafast surface photocurrents in Bi2Se3 with 20 fs resolution, uncovering charge transfer along Se-Bi bonds as the dominant process.

Findings

Ultrafast surface current response dominated by charge transfer along Se-Bi bonds.

Photon-helicity-dependent photocurrents are much smaller than expected.

Time-resolved measurements reveal mechanisms of photocurrent generation in topological insulators.

Abstract

Topological insulators constitute a new and fascinating class of matter with insulating bulk yet metallic surfaces that host highly mobile charge carriers with spin-momentum locking. Remarkably, the direction and magnitude of surface currents can be controlled with tailored light beams, but the underlying mechanisms are not yet well understood. To directly resolve the "birth" of such photocurrents we need to boost the time resolution to the scale of elementary scattering events ($\sim$ 10 fs). Here, we excite and measure photocurrents in the three-dimensional model topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$ with a time resolution as short as 20 fs by sampling the concomitantly emitted broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast surface current response is dominated by a charge transfer along the Se-Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to have orders of magnitude smaller magnitude than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are also of direct relevance for optoelectronic devices based on topological-insulator surface currents.