Wide-Bandgap RBa3(B3O6)3 (R = Nd, Sm, Tb, Dy, and Er) Single Crystals for Ultraviolet Nonlinear Optics

Abstract

Discovery of new materials with enhanced optical properties in the visible and UV-C range can impact applications in lasers, nonlinear optics, and quantum optics. Here, the optical floating zone growth of a family of rare earth borates, RBa₃(B₃O₆)₃ (R = Nd, Sm, Tb, Dy, and Er), with promising linear and nonlinear optical (NLO) properties is reported. Although previously identified to be centrosymmetric, the X-ray analysis combined with optical second harmonic generation (SHG) assigns the noncentrosymmetric P$\overline{6}$ space group to these crystals. Characterization of linear optical properties reveals a direct bandgap of ≈5.61–5.72 eV and strong photoluminescence in both the visible and mid-IR regions. Anisotropic linear and nonlinear optical characterization reveals both Type-I and Type-II SHG phase matchability, with the highest effective phase-matched SHG coefficient of 1.2 pm V⁻¹ at 800-nm fundamental wavelength (for DyBa₃(B₃O₆)₃), comparable to β-BaB₂O₄ (phase-matched d₂₂ ≈ 1.9 pm V⁻¹). Laser-induced surface damage threshold for these environmentally stable crystals is 650–900 GW cm⁻², which is four to five times higher than that of β-BaB₂O₄, thus providing an opportunity to pump with significantly higher power to generate about six to seven times stronger SHG light. Since the SHG arises from disorder on the Ba-site, significantly larger SHG coefficients may be realized by “poling” the crystals to align the Ba displacements. These properties motivate further development of this crystal family for laser and wide bandgap NLO applications.