Flux-Free Growth of β Barium Metaborate Single-Crystal Fibers by Micro-Pulling-Down Technique

β-BBO single-crystal fibers with dimensions up to 15 mm in length and 0.7 mm in diameter devoid of cracks and inclusions were successfully grown by the micro-pulling-down (μ-PD) method directly from a supercooling stoichiometric composition of BBO bath. The suitable pulling rate was 0.1 mm/min. Different morphologies of β-BBO fibers were observed using single-crystal seeds oriented along the c-axis, ã-axis perpendicular to c-axis, and phase-matching directions θ = 22.8° and θ = 47.7°. Strong axial thermal gradients were applied throughout the growth and adjusted for each crystallographic direction. The effects of capillary dimensions on fiber quality were studied. The quality and orientation of the fibers were examined by Raman spectroscopy and polarizing microscopy. The ultraviolet (UV) absorption edge of β-BBO single-crystal fibers was measured at 200 nm. A type I second-harmonic generation was produced with a continuous Nd:YAG laser at 1064 nm on β-BBO fibers oriented according to the phase-matching direction.