Breaking through the ‘‘200 nm deep-ultraviolet wall’’ of phase matching region by cation structural modulation

Adjusting birefringence to push the phase-matching (PM) of nonlinear optical (NLO) crystals blue-shifting to the deep-ultraviolet (DUV) region is the inorganic chemists and material scientists' pursuit, while refractive dispersion as an important but easily overlooked optical properties generally brings the unexpected result. Herein, two new Y-based borates, K₂YB₃O₆F₂ and Rb₂YB₃O₆F₂ have been synthesized by the modulation of A-site cation. In both structures, owing to the structure-directing properties of Y-based polyhedra, the natural advantage of π-conjugated [B₃O₆] groups was pushed to the greatest extent, theoretical calculations and structure analysis revealed that the coplanar arrangement of [B₃O₆] groups results in the large birefringence of K₂YB₃O₆F₂ and Rb₂YB₃O₆F₂. Importantly, the aligned orientation of [B₃O₆] rings makes Rb₂YB₃O₆F₂ possess a strong SHG response of 5.0 × KDP, meanwhile the small dispersion of the refractive indices makes Rb₂YB₃O₆F₂ the first borate crystal containing the strong π-conjugation [B₃O₆] rings with the PM limit breaking through the ‘‘200 nm DUV wall’’. This work indicates that refractive dispersion can be effectively reduced through the cation effect, thereby broadening the PM range.