  We have investigated nonlinear refraction in all-inorganic halide
perovskites, the CsPbBr$_3$ and CsPbBr$_{1.5}$I$_{1.5}$ nanosheet and quantum
dot colloids in toluene, by a novel beam deflection technique using
near-resonant continuous wave lasers. The nonlinear refraction and its
time-evolution measured here have originated from thermal lensing effect.
Nonlinear behaviour of heat transport in terms of intensity dependent thermal
diffusion rate has been observed. Effects of convective heat flow have been
measured at high intensities. Quantum dots have higher nonlinear refraction as
compared to the corresponding nanosheet samples, presumably due to reduced
dimensionality. The effective values of nonlinear refractive index, estimated
here for near-resonant excitations, exceed those reported in the literature for
organic-inorganic hybrid perovskites in the nonresonant excitation regime, by
several orders of magnitude.
