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 ${\mathrm{n}}_2$ of up to ${10}^{-8}{\mathrm{cm}}^2/\mathrm{W}$ in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$. 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 $n_2$. The extracted ${\mathit{n}}_2$ values for ${\mathrm{CsPbBr}}_3$ and ${\mathrm{MAPbBr}}_3$ at $\lambda \approx 1\mu \mathrm{m}$ are $n_2=+2.1\times {10}^{-14}{\mathrm{cm}}^2/\mathrm{W}$ and $n_2=+6\times {10}^{-15}{\mathrm{cm}}^2/\mathrm{W}$, 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.