Development of the new nonlinear optical crystal BaAlBO3F2 (BABF)

A new nonlinear optical (NLO) alumino-borate BaAlBO₃F₂ (BABF) has been discovered. BABF crystal can produce larger SHG coefficients, wide transparent region and moderate birefringence. BABF structure has some similarities with that of the NLO crystal KBe₂BO₃F₂ (KBBF) that possesses the layer structure, but the distance between the interlayers is less in BABF, and the interlayer space is sufficiently filled. The tendency to cleave along the [001] plane in BABF is thus weaker than that of KBBF. This means that BABF crystals would be relatively easier to be grown. In this work, BABF crystal has been grown for the first time using the modified middle-seeded growth (MSSG) method with NaF flux. The crystal growth conditions and growth habits have been investigated. Borates usually have a high viscosity in the high temperature solutions. In this case, the fluoride ions can reduce viscosity by breaking extended -O-B-O- chains to advantage for crystal growth when BABF is melted in the solution. Furthermore, due to the presence of Al element as one of the components of BABF lattice, like Al in K₂Al₂B₂O₇, the physical properties of borate crystal have been greatly improved. BABF crystal exhibits high chemical stability, free of moisture and hygroscopes and has very good mechanical properties. The preliminary optical characterizations have been measured. It exhibits that BABF is an excellent candidate for frequency mixing into the ultraviolet (UV). The details of this work will be presented.