Band resolved imaging of photocurrent in a topological insulator

TL;DR

This study uses time- and angle-resolved photoemission spectroscopy to visualize and understand the microscopic mechanisms of photocurrent generation in the topological insulator Bi$_2$Se$_3$, revealing how resonant optical transitions induce spin-polarized currents.

Contribution

It provides the first microscopic imaging of photocurrent formation in a topological insulator and identifies the specific electronic states involved in photocurrent excitation.

Findings

Photocurrents are generated via resonant optical transitions.

Asymmetric electron populations indicate photocurrent presence.

Spin-orbital textured states are key to photocurrent excitation.

Abstract

We study the microscopic origins of photocurrent generation in the topological insulator Bi$_2$Se$_3$ via time- and angle-resolved photoemission spectroscopy. We image the unoccupied band structure as it evolves following a circularly polarized optical excitation and observe an asymmetric electron population in momentum space, which is the spectroscopic signature of a photocurrent. By analyzing the rise times of the population we identify which occupied and unoccupied electronic states are coupled by the optical excitation. We conclude that photocurrents can only be excited via resonant optical transitions coupling to spin-orbital textured states. Our work provides a microscopic understanding of how to control photocurrents in systems with spin-orbit coupling and broken inversion symmetry.