Spin dependent bandgap renormalization and state filling effect in Bi$_2$Se$_3$ observed by ultrafast Kerr rotation

TL;DR

This study explores ultrafast spin dynamics in Bi2Se3 using Kerr rotation, revealing spin-dependent bandgap renormalization and state filling effects that are key for future ultrafast magneto-optical devices.

Contribution

It provides the first detailed analysis of spin-dependent optical responses in Bi2Se3, linking Kerr rotation changes to bandgap renormalization and state filling effects.

Findings

Kerr rotation angle depends on photon energy near 1.0 eV

Photon-energy dependence explained by spin-dependent refractive index changes

Insights into ultrafast opto-spintronic properties of Bi2Se3

Abstract

We investigate the ultrafast spin dynamics of the prototypical topological insulator $\mathrm{Bi_{2}Se_{3}}$ using time-resolved Kerr-rotation (polarization-change) measurements across near-infrared wavelengths. The Kerr-rotation angle $\Delta \theta_{K}$ of $\mathrm{Bi_{2}Se_{3}}$ was found to significantly depend on photon energy around a resonance transition ($\sim 1.0\ \mathrm{eV}$) of bulk states, as well as the ellipticity of the pump light, in the presence of spin excitation. The observed photon-energy dependence of $\Delta \theta_{K}$ can be well simulated by assuming spin-dependent refractive-index changes in the presence of band-gap renormalization and state-filling effect upon photoexcitation. Our study delivers comprehensive insights into the opto-spintronic properties of bulk $\mathrm{Bi_{2}Se_{3}}$ and the fundamental physical processes underlying polarization changes. These findings are expected to be crucial in developing ultrafast magneto-optical memory devices, which can perform read-and-write operations in the Terahertz regime.