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 mediated by the intrinsic excitonic nonlinearity. 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 MoSe_{2} monolayer to a planar photonic crystal nanocavity. The pronounced excitonic resonance of the monolayer, combined with its seamless integration with the planar nanocavity, facilitates robust exciton-photon hybridization. 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 previously reported thresholds in 2D exciton-polariton systems. Pump-probe spectroscopy reveals that this switching operates on an ultrafast timescale of a few picoseconds. Our Letter establishes a robust platform for nonlinear 2D polaritonics with broad applications in integrated photonic technologies, including all-optical neuromorphic computing and quantum photonic information processing.