| Abstract: Monolayer transition metal dichalcogenides (TMDCs) in the MX2 class (M=Mo, W; X=S, Se, Te) hold exceptionally strong light-matter interaction. The bright emission at the direct-gap from the K/K’ valleys has facilitated many studies on the underlying many-body and valley-spin physics, as well as opened up the prospect for various optoelectronic devices. Interestingly, in addition to these bright exciton states, lower-energy dark excitons are expected to exist in certain TMDC compounds (e.g., WS2 and WSe2), which arise from spin splitting in conduction bands. In this talk, I will present two experimental approaches to probe, and further control these optically-inactive dark excitons in monolayer WSe2. First, I will provide an indirect probe of the dark states by analyzing the temperature dependence of time-resolved photoluminescence. The readily observable bright exciton population exhibits thermal activation, thus allowing us to infer information about the lower-energy dark state. I will then show how we can control the radiative properties and directly brighten these spin-forbidden dark excitons with the application of an in-plane magnetic field. Most significantly, much increased emission lifetime and valley lifetime were exhibited by these dark excitons, thus providing new opportunities for the study of highly correlated Bosonic interactions and of spin-valley physics. In this context, I will present experimental results on how these long-lived dark excitons assist the easy formation of biexcitons. The potentials of these dark states to serve as long-lived valley information carriers will also be discussed. |