Exploring Short-Wavelength Phase-Matching Nonlinear Optical Crystals by Employing KBe2BO3F2 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, M2B4SO10 (M = K, Rb, and Cs) and Rb3B11PO19F3, were successfully synthesized by evolving the KBe2BO3F2 (KBBF) structure into mixed-anionic borosulfate and fluoroborophosphate systems. They display functional [B4SO10]∞ or [B11PO19F3]∞ KBBF-type layers that are composed of [BO3], [BO4], and [SO4] groups or [BO3], [BO4], [BO3F], and [PO4] 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 (λPM) of Rb3B11PO19F3 reaches 180 nm, which implies that Rb3B11PO19F3 can become a prospective deep-UV NLO crystal. In addition, single crystals of K2B4SO10, Rb2B4SO10, and Cs2B4SO10 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.