Generation of Transient Photocurrents in the Topological Surface State of Sb$_{2}$Te$_{3}$ by Direct Optical Excitation with Mid-Infrared Pulses

TL;DR

This study demonstrates that mid-infrared pulses can directly excite the topological surface state of Sb$_{2}$Te$_{3}$, inducing long-lived photocurrents with potential applications in spintronics.

Contribution

It introduces a novel method of using tunable MIR pulses for direct optical excitation of topological surface states, revealing asymmetric transient populations and photocurrent dynamics.

Findings

MIR pulses enable direct excitation across the Dirac point.

Asymmetric transient populations indicate macroscopic photocurrents.

Photocurrents are long-lived and resistant to backscattering.

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.