Dual Action Spectroscopy Exposes the Bright and Dark Excitons of Room-Temperature WSe.

While it has long been accepted that the role of momentum dark excitons in the photoresponse of transition metal dichalcogenides (TMDs) is critical, their weak optical signature inhibits their study through conventional means. Here we expose the room-temperature contributions of both bright and dark excitons to the behavior of a TMD, WSe, from monolayer to multilayer to bulk. To do so, we present dual action spectroscopy, a photocurrent- and luminescence-detected Fourier-transform excitation spectroscopy scheme, to microscopically map the energy landscape of WSe. While bright excitons naturally dominate the luminescence response of the material, dark excitons dominate the current response. Notably, the dark exciton is more accessible than the ground state , while current maps reveal a disparity in the diffusivity of the two states. This work provides the basis for a new, current-detected approach to study the dynamics of dark exciton states across different materials.