Long-lived populations of momentum- and spin-indirect excitons in monolayer WSe2

Abstract

In monolayer WSe2, interactions between the lower-energy momentum- and spin-indirect “dark” excitons and the bright exciton (X) are likely to be significant in determining the optical properties of X at high power, and limit the ultimate exciton densities that can be achieved, yet little is known about them. Here, by employing time-resolved photoluminescence measurements, we demonstrate an efficient population of dark excitons via inter-state conversion between X and the spin-indirect intravalley excitons (D) through spin-flip, and between D and the momentum-indirect intervalley excitons (XK) mediated by the exchange interaction (D+D ←→ XK +XK). Moreover, we observe a persistent redshift of the X exciton on sub-ns timescales due to strong excitonic screening by the long-lived dense XK exciton. Our results provide a new insight into the many-body interactions between bright and dark excitons, and point to a possibility to employ dark excitons for investigating exciton condensation and valleytronics.