Breaking Performance Barriers in KBe2BO3F2 (KBBF) Analogs by Functional Group Self‐Polymerization

Enhancing the conversion efficiency of all‐solid‐state lasers through the rational design of crystal materials with superior linear and nonlinear optical (NLO) properties remains a formidable challenge. Herein, we present a novel approach to optimizing these properties in KBe2BO3F2 (KBBF)‐analog crystals via functional group self‐polymerization. This strategy led to the synthesis of two new optical crystals: noncentrosymmetric CsAs2O3Br and centrosymmetric CsAs4O6Br. By incorporating highly optically active [AsO3]3‐ units into the classical 2D [Be2BO3F]∞‐ framework, we facilitated the self‐assembly of [As2O3]∞ layers, forming a densely packed and highly ordered structure that enhances macroscopic optical activity. CsAs2O3Br exhibited an extraordinary second‐harmonic generation (SHG) response, 20.5 times stronger than KH2PO4 (KDP), while CsAs4O6Br demonstrated exceptional birefringence (0.26 at 546 nm), setting new performance benchmarks among KBBF analogs. Theoretical analyses reveal that these superior properties arise from the efficient alignment and high density of self‐polymerized functional units. This work represents a significant advancement in the design of high‐performance UV NLO materials, particularly for fourth‐harmonic generation, and paves the way for future innovations in photonic technologies, including solar‐blind UV laser systems and advanced photonic devices.