Generation of Transient Photocurrents in the Topological Surface State of Sb${}_2$Te${}_3$ by Direct Optical Excitation with Mid-Infrared Pulses

Abstract

We combine tunable mid-infrared (MIR) pump pulses with time- and angle-resolved two-photon photoemission to study ultrafast photoexcitation of the topological surface state (TSS) of Sb${}_2$Te${}_3$. It is revealed that MIR pulses permit a direct excitation of the unoccupied TSS owing to an optical coupling across the Dirac point. The novel optical coupling provokes asymmetric transient populations of the TSS at $\pm k_{||}$, which mirrors a macroscopic photoexcited electric surface current. By observing the decay of the asymmetric population, we directly demonstrate the dynamics of the long-lived photocurrent and its protection against backscattering. Our discovery promises important advantages of photoexcitation by MIR pulses for spintronic applications.

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• Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3
• Ultrafast surface carrier dynamics in the topological insulator Bi2Te3
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