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 10−8cm2/W in CH3⁢NH3⁢PbBr3. 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≈1µ⁢m are 𝑛2=+2.1×10−14cm2/W and 𝑛2=+6×10−15cm2/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.