Exploring Deep-UV Nonlinear Optical Materials with Enhanced Second Harmonic Generation Response and Birefringence in Fluoroaluminoborate Crystals

Abstract

Deep-UV (DUV) nonlinear optical (NLO) materials are of vital importance to demanding DUV laser applications; however, those with a large NLO coefficient, wide band gap, and sufficient phase-matching ability are rare. In this work, by combining π-conjugated B₃O₆ groups and distort AlO₃F tetrahedra, we successfully designed and synthesized two fluoroaluminoborates, RbAlB₃O₆F (RABF) and Cs_0.5Rb_0.5AlB₃O₆F (CRABF). In addition, the solid solution Cs_<i>x</i>Rb_1-<i>x</i>AlB₃O₆F (0.2 < <i>x</i> < 0.8) polycrystalline samples were obtained by the standard solid-state reaction. It was shown that all these compounds are NLO-active and possess short UV cutoff edges (<200 nm). Interestingly, although they have the same fundamental structural units, their crystal structures and second harmonic generation (SHG) responses are significantly different owing to the cation effect. Theoretical calculations reveal that both B₃O₆ and AlO₃F groups contribute to the SHG responses. Remarkably, it also indicates that RABF exhibits a large birefringence (Δ<i>n</i> = 0.0946 at 1064 nm), which is capable of realizing deep-UV phase matching down to 174 nm. The exploration of fluoroaluminoborates provides a new perspective for searching excellent deep-UV NLO materials.