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

We report the first successful synthesis and characterization of a new family of high-entropy rare earth borate (RⁿBBO) single crystals with compositions R⁵Ba₃(B₃O₆)₃ and R⁶Ba₃(B₃O₆)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-BaB₂O₄ (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.