Enhanced Second-Harmonic Generation of van der Waals CuInP₂S₆ via Pressure-Regulated Cationic Displacement

Design and exploration of high-performance nonlinear optical (NLO) materials have long been sought with the goal of tunable local structures and NLO properties for advanced laser technology. Thus far, the design strategies for second-order NLO materials have been mainly focused on anionic groups that have made great progress in the development of new NLO compounds. However, few studies have focused on the effects of cationic components and their contributions to NLO properties have long been underappreciated and remain unclear. Here, by introducing pressure to continuously tune the Cu displacement, we demonstrate the significant role of the cationic configuration in NLO properties, and a remarkable enhancement by one order of magnitude in second-harmonic generation (SHG) has been achieved at 3.4 GPa in thiophosphate CuInP₂S₆ (CIPS). In situ high-pressure structural characterization indicates that Cu cations move from the van der Waals edge to the cages of S₆ octahedra during compression and subsequently form highly distorted [CuS₆] octahedra at 3.2 GPa. We quantitively demonstrate the contributions of cationic displacement on the octahedral distortion and interband dipole moments, which dominate optical nonlinearity and determine the enhanced SHG. This work deepens the fundamental understanding of the relationship between cationic displacement and SHG properties, thus providing new paths to NLO material design by optimizing the cationic configurations.