Exploring Short-Wavelength Phase-Matching Nonlinear Optical Crystals by Employing KBe 2 BO 3 F 2 as the Template.

Exploration of nonlinear optical (NLO) crystals that are competent in generating short-wavelength ultraviolet (UV, λ ≤ 266 nm, and even deep-UV, λ ≤ 200 nm) coherent light output by direct second harmonic generation (SHG) remains a formidable challenge. Herein, four UV/deep-UV NLO crystals, M 2 B 4 SO 10 (M = K, Rb, and Cs) and Rb 3 B 11 PO 19 F 3 , were successfully synthesized by evolving the KBe 2 BO 3 F 2 (KBBF) structure into mixed-anionic borosulfate and fluoroborophosphate systems. They display functional [B 4 SO 10 ] ∞ or [B 11 PO 19 F 3 ] ∞ KBBF-type layers that are composed of [BO 3 ], [BO 4 ], and [SO 4 ] groups or [BO 3 ], [BO 4 ], [BO 3 F], and [PO 4 ] groups, respectively. Experimental characterization and numerical computation results indicate that these crystals possess exceptional NLO performance, including large SHG responses (0.9-1.7 × KDP at 1064 nm and 0.1-0.3 × β-BBO at 532 nm) and adequate birefringence to fulfill the SHG phase-matching (PM) condition at 266 nm. In particular, the shortest type-I PM wavelength (λ PM ) of Rb 3 B 11 PO 19 F 3 reaches 180 nm, which implies that Rb 3 B 11 PO 19 F 3 can become a prospective deep-UV NLO crystal. In addition, single crystals of K 2 B 4 SO 10 , Rb 2 B 4 SO 10 , and Cs 2 B 4 SO 10 are easily obtained by the high-temperature solution approach. This work will facilitate the discovery of short-wavelength PM NLO crystals by using the KBBF structure as the template.