Strongly nonlinear nanocavity exciton-polaritons in gate-tunable monolayer semiconductors

Achieving optical nonlinearities at ultralow light intensities in solid-state platforms is essential for advancing nonlinear and quantum photonic technologies. A promising approach involves coupling excitons to photons in optical cavities to create exciton-polaritons, where effective photon-photon interactions are enhanced by intrinsic exciton interactions in the material. However, realizing strong polariton nonlinearities within a scalable architecture remains a significant experimental challenge. Here, we demonstrate highly nonlinear two-dimensional exciton-polaritons by coupling a charge-tunable MoSe2 monolayer to a planar photonic crystal nanocavity. The pronounced excitonic resonance of the 2D monolayer, combined with its seamless integration with the planar nanocavity, facilitates robust exciton-photon hybridization, leading to the formation of nanocavity exciton-polaritons. Remarkably, the strong mode confinement of the nanocavity substantially enhances polariton-polariton interactions, enabling all-optical switching of the cavity spectrum with excitation energies as low as 4 fJ -- several orders of magnitude below the previously reported thresholds in 2D exciton-polariton systems. We also observe ultrafast polariton dynamics on the order of a few picoseconds via pump-probe spectroscopy. Our work establishes a scalable platform for nonlinear 2D polaritonics with broad applications in integrated photonic technologies, including all-optical neuromorphic computing and quantum photonic information processing.