Origin of Pressure-Induced Nonlinear Optical Property Enhancement in CsGeCl₃ Perovskite: [GeCl₆] Octahedron Distortion and Band Gap Closing

Second-harmonic generation (SHG) is the wave-wave nonlinear interaction in doubling wave frequencies, spanning from optics, radio, magnetohydrodynamics, and even the middle atmosphere, and enhancing SHG efficiency has attracted broad interest in both fundamental mechanisms and industrial applications. Laser SHG is characterized by a second-order nonlinear susceptibility tensor in nonlinear optical (NLO) crystals, and its intensity is largely related to the lattice distortion. Halide perovskites demonstrate unique photovoltaic and optoelectronic properties due to their high tunability in crystalline and electronic structures, which provides a great platform to investigate their NLO properties. Here, we report a pressure-driven SHG enhancement in all-inorganic lead-free compound CsGeCl₃, which exhibits a considerable SHG intensity with 9.1 times (@ 1030 nm) that of KH₂PO₄ at ambient pressure. Upon compression to 1.07 GPa, the SHG intensity further gains ∼3 times, setting a record-high SHG intensity under high pressure (HP). Concerning the limitations of traditional SHG powder measurements, we developed a new single-crystal angle-resolved polarization strategy to retrieve the intrinsic NLO tensors under HP, confirming pressure-induced SHG enhancement. In situ HP synchrotron X-ray diffraction and Raman scattering reveal a strong correlation between SHG intensity and pressure-induced [GeCl₆] octahedral distortion. Optical absorption measurements show significant band gap closing under compression, further favoring SHG enhancement. First-principles calculations corroborate these findings. Our results offer a tunable and clean strategy to enhance the SHG intensity in halide perovskites and establish a reliable approach for probing pressure-dependent NLO mechanism in general.