Assembly of π-Conjugated [B3O6] Units by Mer-Isomer [YO3F3] Octahedra to Design a UV Nonlinear Optical Material, Cs2YB3O6F2.

Achieving the extreme balance of the key performance requirements is the crucial to breakthrough the application bottleneck for nonlinear optical (NLO) materials. Herein, by assembly of the π-conjugated [B3O6] functional species with the aid of structure-directing property of mer-isomer [YO3F3] octahedra, a new ultraviolet (UV) NLO material, Cs2YB3O6F2 with aligned arrangement of coplanar [B3O6] groups has been synthesized. The polar material exhibits the rare coexistence of the largest second harmonic generation response of 5.6×KDP, the largest birefringence of 0.091 at 532 nm, the shortest Type I phase-matching down to 200.5 nm and deep-ultraviolet transparency among reported acentric rare-earth borates with [B3O6] groups. Remarkably, benefiting from the enhanced bonding force among functional units [B3O6], a firm three-dimensional framework is constructed, which facilitates the growth of large crystals. This can be proved by a block shape crystal with dimensional of 6×5×4 mm3, indicating that it was a promising UV NLO crystal. This work provides a powerful strategy to design UV NLO materials with good performances.

An Effective Strategy for Designing Nonlinear Optical Crystals by Combining the Structure-Directing Property of Oxyfluorides with Chemical Substitution.

Rationally designing a high-performance nonlinear optical (NLO) crystal remains a great ongoing challenge. It involves not only the design of noncentrosymmetric structures but also property optimization. In this communication, a new strategy for effectively designing the NLO crystal has been put forward, that is, using the structure-directing property of oxyfluoride anions to obtain a noncentrosymmetric and polar structure, and then by substitution of IO3 - for isovalent F- anions to further enhance the SHG response. With this strategy, a new iodate fluoride, Ba2 [MoO3 F(IO3 )](MoO3 F2 ) has been successfully designed from the Ba2 (MoO3 F2 )F2 with the cis-directing [MoO4 F2 ]4- groups. It exhibits a large SHG response (≈8×KDP), a wide transparent region (0.30-10.92 µm), a high laser-induced damage threshold (LDT) (88.53 MW cm-2 ), and a large birefringence (≈0.264@532 nm). These indicate Ba2 [MoO3 F(IO3 )](MoO3 F2 ) would be a promising NLO crystal.