Electrically Tunable, Large Excitonic Reflectivity from an Atomically Thin Semiconductor

Two dimensional transition metal dichalcogenide (TMD) monolayers are direct bandgap semiconductors that feature tightly bound excitons. TMDs can be easily integrated with various functional substrates or materials through van der Waals stacking. For these reasons, TMDs are particularly interesting both for novel device applications as well as studies of fundamental physics. In this work, we show that a monolayer of the TMD MoSe₂ encapsulated by hexagonal boron nitride can reflect up to 85% of incident light at the excitonic resonance. We study the root of this high reflectance: the excellent coherence properties of excitons in this atomically thin semiconductor. Moreover, we show that power and wavelength dependent nonlinearities in these systems stem from exciton-based lattice heating and exciton-exciton interactions.