Thermal Nonlinear Refraction in Cesium Lead Halide Perovskite Nanostructure Colloids

Nonlinear refraction in all-inorganic halide perovskites, the CsPbBr3 and CsPbBr1.5I1.5 nanocube and nanosheet colloids in toluene, has been studied by a time-resolved, two-color pump–probe beam-deflection technique using above-bandgap excitation by low-power, continuous-wave (CW) diode lasers, as opposed to high-power pulsed lasers commonly used in nonlinear optical measurements. An enhancement of nonlinear refraction coefficient by eight orders from the values previously reported in the literature in similar materials for below-bandgap excitation has been observed at CW laser power as low as 10 mW. This may facilitate mass-scale deployment of these materials and method to nonlinear optical devices and applications. Nanocubes show higher nonlinear refraction as compared to the corresponding nanosheet samples, presumably due to the reduced dimensionality. The nonlinear refraction and its time-evolution have originated from the thermal lensing effect, which can be attributed to the anharmonic vibration of atomic nuclei. Observation of intensity-dependent thermal diffusion rate indicates nonlinear heat transport at the mesoscopic scale. Effects of convective heat flow have been measured at laser power exceeding 25 mW. These results may inspire further theoretical and experimental studies of thermal transport in the mesoscopic domain.