Design of deep-ultraviolet zero-order waveplate materials using LiB₃O₅ as the template

Using the well-known LiB₃O₅ as the template, by improving the angle between B₂O₅ units in its structure to reduce birefringence, two novel birefringent crystals Li₂MLaB₁₈O₃₀ (M = Rb, Cs) for preparing zero-order wave-plates are obtained by a high-temperature solution method in a closed system. Interestingly, the neighboring B₂O₅ units are aligned almost perpendicularly, making them exhibit tiny birefringence, which is much smaller (0.0032@532 nm and 0.0027@532 nm, respectively) than that of the commercial quartz crystal (Δn ~0.0092@visible region). Moreover, the ultraviolet cutoff edges of Li₂MLaB₁₈O₃₀ (M = Rb, Cs) crystals are below 200 nm, demonstrating that they can be applied as zero-order waveplate materials in the deep-ultraviolet region.