# Helicity dependent photocurrent in electrically gated (Bi,Sb)_2Te_3 thin films

## TL;DR

This study investigates the origin of helicity-dependent photocurrent in (Bi,Sb)_2Te_3 thin films, revealing the circular photo-galvanic effect as the main mechanism and relating it to asymmetric optical transitions, with implications for opto-spintronic devices.

## Contribution

The paper identifies the circular photo-galvanic effect as the primary cause of helicity-dependent photocurrent in topological insulator films and relates it to optical transition asymmetries.

## Key findings

- Circular photo-galvanic effect is the dominant mechanism.
- Photocurrent depends on incidence angle, wavelength, and chemical potential.
- Asymmetric optical transitions explain the directional photocurrent.

## Abstract

Circularly polarized photons are known to generate a directional helicity-dependent photocurrent in three-dimensional topological insulators at room temperature. Surprisingly, the phenomenon is readily observed at photon energies that excite electrons to states far above the spin-momentum locked Dirac cone and the underlying mechanism for the helicity-dependent photocurrent is still not understood. We resolve the puzzle through a comprehensive study of the helicity-dependent photocurrent in (Bi,Sb)_2Te_3 thin films as a function of the incidence angle of the optical excitation, its wavelength, and the gate-tuned chemical potential. Our observations allow us to unambiguously identify the circular photo-galvanic effect as the dominant mechanism for the helicity-dependent photocurrent. Additionally, we use an analytical calculation to relate the directional nature of the photocurrent to asymmetric optical transitions between the topological surface states and bulk bands. The insights we obtain are important for engineering opto-spintronic devices that rely on optical steering of spin and charge currents.