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

Abstract 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 KBe 2 BO 3 F 2 (KBBF)‐analog crystals via functional group self‐polymerization. This strategy led to the synthesis of two new optical crystals: noncentrosymmetric CsAs 2 O 3 Br and centrosymmetric CsAs 4 O 6 Br. By incorporating highly optically active [AsO 3 ] 3− units into the classical 2D [Be 2 BO 3 F] ∞ − framework, we facilitated the self‐assembly of [As 2 O 3 ] ∞ layers, forming a densely packed and highly ordered structure that enhances macroscopic optical activity. CsAs 2 O 3 Br exhibited an extraordinary second‐harmonic generation (SHG) response, 20.5 times stronger than KH 2 PO 4 (KDP), while CsAs 4 O 6 Br 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.