Enhancing Second Harmonic Generation via Hydrostatic Pressure in 1D vdWs Ferroelectric WOCl 4

ABSTRACT Achieving controllable nonlinear optical effects up to the ultimate one‐dimensional (1D) single‐chain limit holds significant promise for high‐density integrated photonics and tunable optoelectronic devices. However, experimental investigations of intrinsic 1D van der Waals (vdWs) ferroelectric nonlinear optical materials with well‐defined single‐chain structures remain unexplored. Here, we experimentally report 1D vdWs ferroelectric WOCl 4 , and investigate its second‐harmonic generation (SHG) response, demonstrating a practical mechanism for enhancing its SHG intensity via hydrostatic pressure. Under a fundamental wavelength of 1034 nm, WOCl 4 exhibits the considerable SHG response, which can be further enhanced via the external pressure around 4.5 GPa, suggesting an effective strategy for boosting nonlinear optical responses in 1D systems. Quantitative analysis reveals SHG susceptibility χ ( 2 ) of WOCl 4 reaches 14.7 pm/V under ambient conditions, increasing remarkably to 54.9 pm/V at 4.48 GPa. Combining in situ pressure‐dependent Raman spectroscopy, first‐principles calculations, we demonstrate that O anions undergo the substantial displacements along W‐O‐W chains under the applied pressure, resulting in the enhanced polar distortions within [WO 2 Cl 4 ] octahedral units and thereby substantially boosting SHG response. This study expands the family of 1D vdWs ferroelectric materials with pronounced optical nonlinearity and demonstrates a feasible route for precisely tuning nonlinear optical properties through lattice strain engineering.