Unusual Nonlinear Absorption Switching in Mixed‐Halide Perovskites by Light‐Induced Halide Segregation

Mixed‐halide perovskites (MHPs) have emerged as important semiconductor materials for optoelectronic devices due to the bandgap tunability by halide composition. However, their device applications are hampered by light‐induced ion migration and halide segregation, leading to the formation of lower bandgap domains with red‐shifted photoluminescence. Previous studies of phase segregation in MHPs have predominantly focused on their linear optical properties. Herein, nonlinear absorption (NLA) properties of MAPbBr2I films before and after femtosecond laser irradiation are investigated using the Micro‐I‐scan technique. An unusual NLA switching from reverse‐saturable absorption (RSA) to saturable absorption (SA) under intense laser illumination is found. It is unveiled that the initial RSA originates from two‐photon absorption in uniform bromide‐rich regions of as‐prepared film whereas the SA occurs due to state filling effect in the newly generated low bandgap I‐rich phases under laser irradiation. The switching threshold and effective NLA coefficient are highly dependent on the distribution of halide ions, which is controllable through the laser illumination time. Furthermore, an all‐optical logic gate scheme is demonstrated based on the unusual NLA switching. The results not only unveil that halide segregation results in NLA switching in MHPs but also pave the way for the realization of MHPs‐based nonlinear photonic devices.