Third Harmonic Generation in Lead Bromide Perovskites*

Lead halide perovskites have garnered a lot of attention both in displays and photovoltaics. Along with their remarkable linear optical properties, these materials provide a testbed for exploring nonlinear optical properties. As in the case with several of the 3D lead halide perovskite systems, the underlying centrosymmetric crystal structure precludes the phenomenon of second harmonic generation. However, the third and higher-order harmonic generation are allowed. In this work, we probe the third harmonic generation (THG) from CsPbBr₃ nanocrystals (NCs), synthesized via hot injection method, and compare it to the THG from CsPbBr₃ NCs with Ruddlesden-Popper planar faults (RP-CsPbBr₃), formed via post-synthetic fusion-growth. The THG from CsPbBr₃ NCs is negligible compared with that of RP-CsPbBr₃ NCs within a wide range of femtosecond excitation wavelengths: 1100–1400 nm. The THG efficiency of a thin film of RP-CsPbBr₃ is found to be at least three times larger than the value from a single crystal of methylammonium lead bromide (MAPbBr₃). By comparing with MAPbBr₃, we obtain χ³_eff for a thin film of RP-CsPbBr₃ to be of the order of 10⁻¹⁷m²V⁻².