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.