Valley-Polarized Exciton Dynamics in Exfoliated Monolayer WSe2

Semiconducting transition-metal dichalcogenide monolayers have emerged as promising candidates for future valleytronics-based quantum information technologies. Two distinct momentum states of tightly bound electron-hole pairs in these materials can be deterministically initialized via irradiation with circularly polarized light. Here we investigate the ultrafast dynamics of such a valley polarization in monolayer tungsten diselenide by means of time-resolved Kerr reflectometry. The observed Kerr signal in our sample stems exclusively from charge-neutral excitons. Our findings support the picture of a fast decay of the valley polarization of bright excitons due to radiative recombination, intraconduction-band spin-flip transitions, intervalley-scattering processes, and the formation of long-lived valley-polarized dark states.