Robust Free Excitons in CH₃NH₃PbI₃ Halide Perovskite Revealed by Four‐Wave Mixing

Abstract Direct bandgap semiconductors possess a unique attribute: the presence of a free exciton state with a high total oscillator strength. This property makes them highly promising for applications in ultrafast optical signal processing and optical computing. One such protocol for optical computing is based on four‐wave mixing (FWM). In this study, the nonlinear optical effect in polycrystalline thin films of halide perovskite MAPbI₃ (MA⁺ = CH₃NH) at low temperatures is demonstrated. Through analyzing the spectroscopy of the FWM signal, studying the photoluminescence excitation spectra, and comparing the findings with results from MAPbI₃ single crystals, it has been discovered that the strongest nonlinear response is observed at the free exciton resonance and in the region of shallow defect states. Surprisingly, the presence of FWM in cross‐linear excitation geometry has been observed, indicating potential involvement of other nonlinear or many‐body effects. The observations of FWM with free excitons even in highly defective MAPbI₃ thin films demonstrate the robustness of the exciton resonance and highlight the practical prospects for utilizing this material in optical computing.