No extraordinary χ(3) in lead-halide perovskites: placing an upper bound on Kerr nonlinearity by means of time-resolved interferometry

Lead halide perovskites have recently been reported to demonstrate an exceptionally high nonlinear (Kerr) refractive index n₂ of up to 10^-84pt0excm²/W in CH₃NH₃PbBr₃. Other researchers, however, observe different, substantially more conservative numbers. In order to resolve this disagreement, the nonlinear Kerr index of a bulk sample of lead halide perovskite was measured directly by means of an interferometer. This approach has many advantages as compared to the more standard z-scan technique. In particular, this method allows studying the induced changes to the refractive index in a time-resolved manner, thus enabling to separate the different contributions to n₂. The extracted n₂ values for CsPbBr₃ and MAPbBr₃ at λ≈10.16em0exm are n₂=+2.1×10^-144pt0excm²/W and n₂=+6×10^-154pt0excm²/W, respectively. Hence, these values are substantially lower than what has been indicated in most of the previous reports, implying the latter one should be regarded with great care.