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

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 ${p}_{x,y}\ensuremath{\leftrightarrow}{p}_{z}$ 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 $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ theory using first-principles density functional theory we estimate the absorption for ${\ensuremath{\sigma}}^{\ifmmode\pm\else\textpm\fi{}}$ circularly polarized photons in the monolayer as $\ensuremath{\sim}1.5$%, which saturates to $\ensuremath{\sim}0.3$% 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.