Long-lived populations of momentum- and spin-indirect excitons in monolayer WSe$_2$

TL;DR

This study reveals long-lived populations of dark excitons in monolayer WSe2, demonstrating their interactions with bright excitons and potential for exciton condensation and valleytronics applications.

Contribution

It provides the first detailed observation of long-lived dark exciton populations and their interactions with bright excitons in monolayer WSe2.

Findings

Long-lived dense populations of dark excitons were observed.

Efficient inter-state conversion between exciton species was demonstrated.

Persistent redshift indicates strong excitonic screening and interactions.

Abstract

Monolayer transition metal dichalcogenides are a promising platform to investigate many-body interactions of excitonic complexes. In monolayer tungsten diselenide, the ground-state exciton is dark (spin-indirect), and the valley degeneracy allows low-energy dark momentum-indirect excitons to form. Interactions between the dark exciton species and the optically accessible bright exciton (X) are likely to play significant roles in determining the optical properties of X at high power, as well as limiting the ultimate exciton densities that can be achieved, yet so far little is known about these interactions. Here, we demonstrate long-lived dense populations of momentum-indirect intervalley ($X_K$) and spin-indirect intravalley (D) dark excitons by time-resolved photoluminescence measurements (Tr-PL). Our results uncover an efficient inter-state conversion between X to D excitons through the spin-flip process and the one between D and $X_K$ excitons mediated by the exchange interaction (D + D to $X_K$ + $X_K$). Moreover, we observe a persistent redshift of the X exciton due to strong excitonic screening by $X_K$ exciton with a response time in the timescale of sub-ns, revealing a non-trivial inter-state exciton-exciton interaction. Our results provide a new insight into the interaction between bright and dark excitons, and point to a possibility to employ dark excitons for investigating exciton condensation and the valleytronics.