Colossal enhancement of nonlinear optical properties through chemical disorder in borates

Discovery of materials with exceptional frequency-conversion efficiency remains a central challenge in optoelectronics. In the wide-bandgap regime (crucial for ultraviolet and high-power applications), only a handful of crystals, such as β-BaB₂O₄(BBO), CsB₃O₅(CBO), CsLiB₆O₁₀(CLBO), and KDP, are routinely employed, yet each suffers from limited stability, modest nonlinear susceptibility, or low laser-induced damage thresholds. We have recently discovered this high-entropy borate system that overcomes these constraints and exhibits two orders of magnitude increase in optical second-harmonic generation (SHG) relative to commercial BBO. Using configurational entropy as a tuning knob, we have obtained higher optical transparency and broadband photoluminescence in both the visible and near-infrared wavelength ranges, with emission bands at 605, 705, 813, 910, and 1030 nm. Coupled with high-resolution SHG microscopy for phase identification and discovery, we have explored the structural and crystallographic origins of this phase through high-resolution hard-x-ray nanoprobe and 4D-STEM analysis, which reveal a structural distortion that lowers the crystallographic symmetry and is responsible for this enhancement. This work illuminates the promise of a high entropy synthesis strategy for designing next-generation optoelectronic materials that combine increased transparency, strong broadband luminescence, and enhanced nonlinear response in a single platform.