Enhanced Third Harmonic Generation in Lead Bromide Perovskites with Ruddlesden-Popper Planar Faults.

Lead halide perovskites provide a test bed for exploring nonlinear optical properties. Although the underlying centrosymmetric crystal structure of 3D lead halide perovskites precludes the phenomenon of second harmonic generation, the third and higher-order harmonic generation are allowed. In this work, we probe the third harmonic generation (THG) from CsPbBr nanocrystals (NCs) and compare it to the THG from CsPbBr NCs with Ruddlesden-Popper planar faults (RP-CsPbBr), formed via postsynthetic fusion-growth. The THG from CsPbBr NCs is negligible compared with that of RP-CsPbBr NCs within a wide range of femtosecond excitation wavelengths. We further compare the THG from a thin film of RP-CsPbBr with that of a single crystal of methylammonium lead bromide (MAPbBr). The THG efficiency of RP-CsPbBr is found to be three times greater than that of MAPbBr. An effective third-order susceptibility of the order of 10 m V is obtained for a RP-CsPbBr film, opening up the prospect of inorganic halide perovskite NCs with planar defects for a range of nonlinear optical applications.