Regulating B‐O Arrangement Boosts Anisotropy in a Simple Rubidium Borate, Leading to a High‐Performance UV NLO Crystal

Ultraviolet nonlinear optical (UV NLO) crystals are key components in solid‐state lasers. However, commercialized crystals remain limited, especially in exploring new materials within chemically simple systems poses significant challenges. Here, using the classic LiB 3 O 5 as a template and rearranging the [B 3 O 7 ] units, thereby the birefringence can be enhanced to achieve a shorter phase‐matching wavelength, which leads to the successful synthesis of a competitive 266 nm UV NLO crystal RbB 3 O 5 ‐ Cc . The crystal exhibits excellent NLO properties: a short deep‐UV cut‐off edge (<182 nm), a large second harmonic generation response (1.4 × KH 2 PO 4 ), and a sufficient birefringence (0.067 at 1064 nm), with the shortest phase‐matching wavelength down to 220 nm. This study highlights the achievement of enhanced structural integrity and improved material properties through structural modification, which provides new insights into the exploration of high‐performance UV NLO crystals, and boosts confidence in developing new functional materials in simple systems.