Selective probing of photo-induced charge and spin dynamics in the bulk and surface of a topological insulator

TL;DR

This study uses time-resolved optical pump-probe techniques to distinguish and analyze photo-induced charge and spin dynamics in the bulk and surface of the topological insulator Bi2Se3, revealing different relaxation times due to strong spin-orbit coupling.

Contribution

It introduces a novel rotational anisotropy analysis method to separately resolve spin and charge relaxation processes in topological insulators.

Findings

Transient net spin density can be optically induced in bulk and surface

Spin and charge relax on very different time scales

Spin de-polarization, intraband cooling, and interband recombination are separately characterized

Abstract

Topological insulators possess completely different spin-orbit coupled bulk and surface electronic spectra that are each predicted to exhibit exotic responses to light. Here we report time-resolved fundamental and second harmonic optical pump-probe measurements on the topological insulator Bi2Se3 to independently measure its photo-induced charge and spin dynamics with bulk and surface selectivity. Our results show that a transient net spin density can be optically induced in both the bulk and surface, which may drive spin transport in topological insulators. By utilizing a novel rotational anisotropy analysis we are able to separately resolve the spin de-polarization, intraband cooling and interband recombination processes following photo-excitation, which reveal that spin and charge degrees of freedom relax on very different time scales owing to strong spin-orbit coupling.