Ultrafast Dynamics of Spin Generation and Relaxation in Layered WSe.

We investigated the build-up and relaxation processes of spin-polarized A- and B-exciton dynamics in monolayer, bilayer, and bulk WSe using helicity-resolved two-color pump-probe spectroscopy. Substantial spin polarization was confirmed in bulk crystals, though the spin polarization degree of A excitons decreased from monolayer to bulk. However, the spin polarization of A excitons almost vanished in all different layered-flakes when resonantly pumping the B-exciton transition, owing to the dominant role of interexciton transfer. When resonantly pumping the A-exciton transition, the spin polarization of the up-converted B excitons was inverted in all layers because of the efficient Dexter-like coupling and phonon-assisted scattering. The same short spin relaxation time (1.8 ± 0.2 ps) of A excitons was found for all studied flakes in the subsequent spin depolarization processes, which was ascribed to the active electron-phonon scattering resulting from the intrinsic small conduction-band spin-orbit coupling splitting in layered WSe.