Rational Design via Synergistic Combination Leads to an Outstanding Deep-Ultraviolet Birefringent LiNaBO Material with an Unvalued BO Functional Gene.

Birefringent materials, the key components in modulating the polarization of light, are of great importance in optical communication and the laser industry. Limited by their transparency range, few birefringent materials can be practically used in the deep ultraviolet (DUV, λ < 200 nm) region. Different from the traditional BO- or BO-based DUV birefringent crystals, we propose a new functional gene, the BO unit, for designing birefringent materials. Excitingly, the synergistic combination of LiBO and NaBO generates a new compound, LiNaBO, with enhanced optical properties. The LiNaBO crystal with a size of up to 35 × 15 × 5 mm was grown by the top-seeded solution growth (TSSG) method, and its physicochemical properties were systematically characterized. LiNaBO features a large amount of birefringence (0.095@532 nm), a short DUV cutoff edge (181 nm) with a high laser-induced damage threshold (LDT, 7.5 GW/cm @1064 nm, 10 ns), favorable anisotropic thermal expansion (α/α = 5.6), and the lowest crystal growth temperature (<609 °C) among the commercial birefringent crystals. Moreover, the influences of the BO structural configurations on the optical anisotropy were explored. The fascinating experimental results will provide a prominent DUV birefringent crystal and an effective synthesis strategy, which can facilitate the design of DUV birefringent materials.