Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3.

Lukas Braun,Martin Wolf,Luca Perfetti,Thomas Schumann,Markus Münzenberg,Andrzej Hruban,Tobias Kampfrath,Gregor Mussler,Marcin Konczykowski

doi:10.1038/ncomms13259

Abstract

Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (∼10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi2Se3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se–Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents.

Highlights

Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum
We observe that currents depending on the pump helicity are orders of magnitude smaller than expected from the photocurrent generation scenario based on asymmetric depopulation of the Dirac cone[6]
Summarizing our results, we have shown that our ultrabroadband THz emission data are fully consistent with the notion that (i) the photocurrent Jx arises from an instantaneous photoinduced shift of charge density by B1 Å in an B2 nm thick surface region of Bi2Se3

Summary

Introduction

Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. We observe that currents depending on the pump helicity are orders of magnitude smaller than expected from the photocurrent generation scenario based on asymmetric depopulation of the Dirac cone[6] This remarkable result suggests a strong mutual cancellation of the contributions of the various optical transitions, much reduced matrix elements for surface-to-bulk transitions and/or relatively small pump-induced changes in the electron band velocity. For the first time, we observe a new type of photocurrent, a surface shift current, which originates from an instantaneous displacement of electron density along the Se–Bi bond This charge transfer is localized in a surface region of B2 nm thickness, the natural confinement scale of topological surface states. The instantaneous electric field generated by the shift current could be used to drive highly spin-polarized THz electric currents at the TI surface along an tunable direction

Methods

Results

Conclusion