Brightening of spin- And momentum-dark excitons in transition metal dichalcogenides

Abstract

Monolayer transition metal dichalcogenides (TMDs) have been in focus of current research, among others due to their remarkable exciton landscape consisting of bright and dark excitonic states. Although dark excitons are not directly visible in optical spectra, they have a large impact on exciton dynamics and hence their understanding is crucial for potential TMD-based applications. Here, we study brightening mechanisms of dark excitons via interaction with phonons and in-plane magnetic fields. We show clear signatures of momentum- and spin-dark excitons in WS2, WSe2 and MoS2, while the photoluminescence of MoSe2 is only determined by the bright exciton. In particular, we reveal the mechanism behind the brightening of states that are both spin- and momentum-dark in MoS2. Our results are in good agreement with recent experiments and contribute to a better microscopic understanding of the exciton landscape in TMDs.