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

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 Cs0.5Rb0.5AlB3O6F (CRABF). In addition, the solid solution
CsxRb1–xAlB3O6F (0.2 < x < 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 (Δn = 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.