Spin-orbit coupling, optical transitions, and spin pumping in mono- and few-layer InSe

TL;DR

This paper demonstrates that spin-orbit coupling in InSe enables unique optical transitions with polarized light, allowing for spin-selective excitation and potential nuclear spin polarization in mono- and few-layer structures.

Contribution

It reveals how SOC enables optical transitions in InSe that couple with polarized light and facilitates spin pumping, a novel mechanism in this material.

Findings

Optical transition across the band gap couples with in-plane polarized light due to SOC.

Absorption for circularly polarized light is approximately 1.5% in monolayers, decreasing in thicker films.

Circularly polarized light can selectively excite spin-polarized electrons, enabling spin pumping.

Abstract

We show that spin-orbit coupling (SOC) in InSe enables the optical transition across the principal band gap to couple with in-plane polarized light. This transition, enabled by $p_{x,y}\leftrightarrow p_z$ hybridization due to intra-atomic SOC in both In and Se, can be viewed as a transition between two dominantly $s$- and $p_z$-orbital based bands, accompanied by an electron spin-flip. Having parametrized $\mathbf{k\cdot p}$ theory using first principles density functional theory we estimate the absorption for $\sigma^{\pm}$ circularly polarized photons in the monolayer as $\sim 1.5\%$, which saturates to $\sim 0.3\%$ in thicker films ($3-5$ layers). Circularly polarized light can be used to selectively excite electrons into spin-polarized states in the conduction band, which permits optical pumping of the spin polarization of In nuclei through the hyperfine interaction.