A microfluidic platform integrated with tapered optical fiber for studying resonant properties of compact high index microspheres

In this work we designed and tested a micro fluidic platform integrated with a tapered fiber for studying resonant properties of a variety of microspheres sizes from 3 μm to 25 μm with different index of refraction from 1.47 to 2.1 in water and air environments. Experiments are performed by reattaching the microspheres along the tapered fibers with the diameter varying from 1 μm to 7 μm. Although the quality factors of whispering gallery modes (WGMs) are usually degraded for compact cavities immersed in liquid due to the reduced index contrasts, we show that this problem can be solved by using cavities made from high index materials. We demonstrated loaded Q-factors approaching ~10⁴ for coupling to WGMs in compact (as small as 4 - 5 μm in diameter) barium titanate glass microspheres with refractive indices 1.9 and 2.1. The microfluidic platform developed in this work can be used for studying resonant optical forces exerted on compact microspheres in a liquid as well as for developing compact microsphere resonator sensor devices.