Unraveling photoinduced spin dynamics in topological insulator Bi2Se3

TL;DR

This study uses ultrafast optical spectroscopy to explore how circularly polarized light induces spin polarization in the topological insulator Bi2Se3, revealing distinct spin relaxation times and mechanisms involving surface and bulk states.

Contribution

It demonstrates that spin polarization can be generated via interband transitions in Bi2Se3 and identifies two different spin relaxation times linked to surface-bulk state coupling.

Findings

Spin polarization can be generated using circularly polarized light.

Two distinct spin relaxation times are observed (~25 fs and ~300 fs).

Spin dynamics are influenced by phonon modes in surface and bulk states.

Abstract

We report on time-resolved ultrafast optical spectroscopy study of the topological insulator (TI) Bi$_2$Se$_3$. We unravel that a net spin polarization can not only be generated using circularly polarized light via interband transitions between topological surface states (SSs), but also via transitions between SSs and bulk states. Our experiment demonstrates that tuning photon energy or temperature can essentially allow for photoexcitation of spin-polarized electrons to unoccupied topological SSs with two distinct spin relaxation times ($\sim$25 fs and $\sim$300 fs), depending on the coupling between SSs and bulk states. The intrinsic mechanism leading to such distinctive spin dynamics is the scattering in SSs and bulk states which is dominated by $E_g^2$ and $A_{1g}^1$ phonon modes, respectively. These findings are suggestive of novel ways to manipulate the photoinduced coherent spins in TIs.