Na₃Ba₂(B₃O₆)₂F: Next Generation of Deep-Ultraviolet Birefringent Materials

Birefringent materials are of great importance in optical communication and the laser industry, as they can modulate the polarization of light. Limited by their transparency range, few birefringent materials, except α-BaB₂O₄ (α-BBO), can be practically used in the deep ultraviolet (UV) region. However, α-BBO suffers from a phase transition and does not have enough transparency in the deep UV region. By introducing the relatively small alkali metal Na⁺ cation and the F^– anion to keep the favorable structural features of α-BBO, we report a new birefringent crystal Na₃Ba₂(B₃O₆)₂F (NBBF), which has the desirable optical properties. NBBF not only maintains the large birefringence (Δ<i>n</i> = <i>n</i>_o – <i>n</i>_e = 0.2554–0.0750 from 175 nm to 3.35 μm) and extends its UV cutoff edge to 175 nm (14 nm shorter than α-BBO) but also eliminates the phase transition and has the lowest growth temperature (820 °C) among birefringent materials. These results demonstrate that NBBF is an attractive candidate for the next generation of deep UV birefringent materials