Beam Routing through Excitons in Transition Metal Dichalcogenide Monolayers

Abstract

Routing light at the nanoscale typically relies on nanostructured surfaces to imprint directionality on the emission. Using low-temperature, angle-resolved cathodoluminescence spectroscopy, we show that the intrinsic excitonic transitions of a semiconductor can themselves produce routed emission. We probe monolayers of WSe$_2$, MoSe$_2$, and MoTe$_2$ and resolve the excitonic species of monolayer WSe$_2$ -- the bright exciton, the trion, and the spin-forbidden dark exciton -- through their distinct angular emission profiles. While the in-plane transition dipoles of the bright exciton and trion radiate predominantly toward the surface normal, the out-of-plane dipole of the dark exciton, inaccessible under normal-incidence optical excitation, produces a directional emission channel at large angles. We further tune the balance between neutral and charged exciton emission through the local dielectric environment. Our results establish dark excitons in TMD monolayers as a platform for directional light emission in compact photonic architectures without additional nanostructuring.