In monolayer WSe2, the ground-state exciton is dark (D exciton, spin-indirect), and the valley degeneracy allows low-energy dark momentum-indirect excitons (XK exciton) to form. Interactions between the dark excitons and the optically accessible bright exciton (X) are likely to determine X exciton’s optical properties at high power and limit the ultimate exciton densities be achieved. However, so far, little is known about these interactions.
Here, we show long-lived dense populations of the XK and the D excitons by performing time-resolved photoluminescence spectroscopy using a streak camera. Our results uncover an efficient inter-state conversion between X to D excitons through the spin-flip process. Surprisingly, we also observe a prominent conversion between dark excitons mediated by a four-particle interaction (D + D ↔ XK + XK). The latter interaction is superlinear to the exciton densities and responsible for the XK exciton population at high density. Moreover, we found the long-lived XK exciton can induce a persistent redshift of X exciton, indicating non-trivial exciton-exciton interactions. Our results clarify bright and dark excitons’ interactions and further envision the Bose-Einstein condensation of dark excitons.
About the presenter
Dr Shao-Yu Chen is a research fellow working with Prof Michael Fuhrer at Monash University to realising Bose-Einstein Condensation of tightly bound excitons in 2D semiconductors. He has extensive research experiences on transport and optical measurement on 2D materials including graphene and transition metal dichalcogenides (TMDs). In particular, he has investigated strong Coulomb interaction and many-body correlation in atomically-thin electronic systems through luminescence and Raman scattering measurements. Shao-Yu works within FLEET’s Research theme 2: exciton superfluids and Enabling technologies A (atomically-thin materials) and B (nano-device fabrication).