Fiber coupling to BaTiO₃ glass microspheres in an aqueous environment

Compact microspheres with high-quality (Q) whispering gallery modes are required for many applications involving liquid immersion, such as sensing nanoparticles and studying resonant radiative pressure effects. We show that high-index (1.9 and 2.1) barium titanate glass (BTG) microspheres are perfect candidates for these applications due to their high-Q (∼10⁴ in the 1100-1600nm range) resonances evanescently excited in spheres with diameters of 4-15 μm. By reattaching the spheres at different positions along a tapered optical fiber, we show that the coupling constant exponentially increases with thinner fiber diameters. We demonstrate the close to critical coupling regime with intrinsic Q = 3 × 10⁴ for water immersed 14 μm BTG spheres.