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. Yet, so far, continuous-wave (CW) nonlinearities have remained elusive. Here we demonstrate CW nonlinear phenomena in a CsPbBr₃ 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, which may indicate a phase transition in CsPbBr₃. Our results position CsPbBr₃ cavities as a suitable platform for nonlinear optics, offering strong and tuneable CW nonlinearity and birefringence. Moreover, our approach to uncover signatures of a phase transition of matter via optical hysteresis measurements is promising for exploring strongly correlated states of light–matter systems.