Design of novel deep-UV nonlinear optical materials with one-dimensional functional module [BO2]∞ chain and fluorine-driven short phase-matching

Design of deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials is urgently needed owing to the lack of high-performance crystals to meet the contradictory criteria of deep-UV transparency, strong NLO response and moderate birefringence to reach deep-UV phase-matching. In this work, a "highly-polymerized units optimizing electron delocalization" strategy was proposed to explore novel types of deep-UV NLO anionic functional module and design new materials. Two novel NLO systems with new NLO anionic functional module-one dimensional [BO2]∞ chains were successfully designed. In total, four LiBO2 (I-IV) and five Li2BO2F (I–V) structures with good thermodynamically and dynamically stable/metastable were obtained by using evolutionary algorithm crystal structure prediction techniques. All these structures contain the [BO2]∞ infinite chains and have deep-UV transparency (< 177.3 nm). LiBO2 (II, IV) and Li2BO2F (I, IV-V) exhibit large NLO response (2.0–2.3 × KH2PO4) and large birefringence (0.093–0.146 @1064 nm). Remarkably, Li2BO2F–I possesses an extremely short phase-matching wavelength about 159 nm, which is beneficial from the introduction of fluorine to maintain the superior functional module [BO2]∞ and optimize the optical properties. The results indicate that [BO2]∞ can be as a new deep-UV NLO functional module, this work provides an insight into the design of new deep-UV compound with outstanding NLO performances.