Harnessing highly efficient coherent polariton parametric emission in quantum confined perovskite microcavities

Microcavity exciton polaritons emerge as a versatile platform for nonlinear optical effects thanks to the unique dispersion that gives access to a manifold of energy and wavevector conserved processes. Among them energy-degenerate optical parametric oscillation with balanced signal and idler emission on two opposite angles appears to be ideal candidate for coherent quantum applications, such as generating correlated photons, and creating supersolid phase of matter. However, efficient generation of parametric emission while preserving strong coherence persists as a critical challenge mainly due to the lack of strong parametric interactions compared to the simultaneous interaction with relaxation channels. Here in this work, we report highly efficient parametric emission with sub-1 meV linewidth from quantum confined microcavities, enabling the first observation of strong phase coherence between the two parametric species in momentum space. Our systematic analysis reveals that a quantum size effect in our confined microcavities as the microscopic mechanism for the observed anomalous enhancement of parametric scattering when system size scales down, highlighting the important role of quantum confinement and disorder in triggering exotic parametric interactions and applications for microcavity polaritons. Finally, we demonstrate the emergence of a polariton supersolid phase at room temperature in a strongly coherent parametric oscillator, characterized by a periodic density modulation in real space that indicates the breaking of translational symmetry.