Atomically thin semiconductors as nonlinear mirrors

We show that a transition-metal dichalcogenide monolayer with a radiatively broadened exciton resonance would exhibit perfect extinction of the transmitted field. This result holds for s-or p-polarized weak resonant light fields at any incidence angle, due to the conservation of the in-plane momentum of excitons and photons in a flat defect-free two-dimensional crystal. In contrast to extinction experiments with single quantum emitters, exciton-exciton interactions lead to an enhancement of reflection with increasing power for incident fields that are blue detuned with respect to the exciton resonance. By taking into account quantum fluctuations, we show that the extinction of such a nonlinear mirror is limited; for light intensities leading to maximal reflection, the transmitted field is in a multimode squeezed state.