Infrared/Terahertz Spectra of the Photogalvanic Effect in (Bi,Sb)Te based Three Dimensional Topological Insulators

TL;DR

This study investigates the infrared and terahertz spectra of photocurrents in (Bi,Sb)Te topological insulators, revealing the mechanisms of the linear and circular photogalvanic effects and their relation to surface and bulk electronic states.

Contribution

It provides a systematic spectral analysis of the photogalvanic effects in (Bi,Sb)Te topological insulators across a wide frequency range, linking spectral features to electronic transitions and carrier dynamics.

Findings

Photocurrents are caused by the linear photogalvanic effect in surface states.

Spectra reveal free carrier Drude-like absorption at low frequencies.

Enhanced photogalvanic effect observed at 30-60 THz due to surface-to-bulk electron excitation.

Abstract

We report on the systematic study of infrared/terahertz spectra of photocurrents in (Bi,Sb)Te based three dimensional topological insulators. We demonstrate that in a wide range of frequencies, ranging from fractions up to tens of terahertz, the photocurrent is caused by the linear photogalvanic effect (LPGE) excited in the surface states. The photocurrent spectra reveal that at low frequencies the LPGE emerges due to free carrier Drude-like absorption. The spectra allow to determine the room temperature carrier mobilities in the surface states despite the presents of thermally activate residual impurities in the material bulk. In a number of samples we observed an enhancement of the linear photogalvanic effect at frequencies between 30{\div}60 THz, which is attributed to the excitation of electrons from helical surface to bulk conduction band states. Under this condition and applying oblique incidence we also observed the circular photogalvanic effect driven by the radiation helicity.