Continuous-Wave Nonlinear Polarization Control and Signatures of Criticality in a Perovskite Cavity

Halide perovskites have emerged as promising photonic materials for fundamental physics studies and technological applications. Their potential for nonlinear optics has also drawn great interest recently; yet, to date, continuous-wave (CW) nonlinearities have remained elusive. Here we demonstrate CW nonlinear phenomena in a CsPbBr$_3$ perovskite cavity. We first demonstrate optical bistability -- the hallmark of single-mode coherent nonlinear optics. Next we exploit the interplay of nonlinearity and birefringence to demonstrate nonlinear control over the polarization of light. Finally, by measuring the optical hysteresis of our cavity as a function of temperature, we find a dramatic enhancement of the nonlinearity around 65 K. This enhancement is indicative of a phase transition in CsPbBr$_3$. Our results position CsPbBr$_3$ cavities as an exceptional platform for nonlinear optics, offering strong CW nonlinearity and birefringence which are furthermore tunable. In addition, our approach to uncover a phase transition of matter via optical hysteresis measurements is promising for exploring strongly correlated states of light-matter systems.