High Entropy Wide‐Bandgap Borates with Broadband Luminescence and Large Nonlinear Optical properties

ABSTRACT We report the first successful synthesis and characterization of a new family of high‐entropy rare earth borate ( R ⁿ BBO) single crystals with compositions R ⁵ Ba ₃ (B ₃ O ₆ ) ₃ and R ⁶ Ba ₃ (B ₃ O ₆ ) ₃ ( R = Nd, Tb, Sm, Dy, Gd, Yb, Er). Using configurational entropy as a tuning knob, these systems have been grown as large, highly crystalline boules that exhibit a bandgap of ≈5 eV and significantly enhanced optical transparency (20–50%) over singlecomponent systems. The presence of multiple rare‐earth elements results in broadband photoluminescence in both the visible and the near‐infrared wavelength ranges, with co‐existing emission bands at 605, 705, 813, 910, and 1030 nm. Further, broken inversion symmetry enables optical second‐harmonic generation (SHG) with potential for both type‐I and type‐II phase matching. Our highest observed effective phase‐matched SHG coefficient of ≈ 2.1 pm V ⁻¹ at 800–400 nm wavelength conversion is 20% better than the commercial β‐BaB ₂ O ₄ (BBO), while its laser‐induced surface damage threshold is 5‐6 × larger for 100 fs 800 nm pulse, enabling potentially an order of magnitude improvement in the frequency conversion efficiency. This work illuminates the promise of 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.