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) <-> 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 k . p theory using first-principles density functional theory we estimate the absorption for sigma(+/-) circularly polarized photons in the monolayer as similar to 1.5%, which saturates to similar to 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.