Excitonic versus Free-Carrier Contributions to the Nonlinearly Excited Photoluminescence in CsPbBr3 Perovskites

Nonlinear optical absorption of light and efficient luminescence in halide perovskites are important photophysical processes, which can be employed for efficient light conversion from infrared into visible, and result in development of microlasers pumped by infrared light, as well as highly efficient broadband upconverters of light. One of the most critical parameters in these processes is the order of nonlinearity, which allows for prediction of the output emission power for various systems. Herein, we show theoretically and experimentally how the order of nonlinearly excited photoluminescence in halide perovskites is governed by an interplay between contributions from free carriers and excitons. Our generalized theoretical model is confirmed by experiments where we reveal a strong dependence of the nonlinearity order in CsPbBr3 films and nanocrystals on temperature, excitation wavelength, and thickness and size. By incorporating CsPbBr3 nanocrystals into optically resonant porous calcium carbonate (CaCO3) microspheres, we demonstrate how the developed approach can be applied in nonlinear nanophotonics, where the information about the order of the nonlinearity is crucial.