Abstract
Optical microsphere resonators are widely applied in various fields including, optical communications, researches on nonlinear optical effect, dye lasers, and label free detection for biosensing, due to the high quality factor and small mode volume. However, to effectively excite the microsphere resonator is challenging since the external light should be coupled to the resonant modes, or called whispering gallery modes (WGM), with a precisely controlled distance. In this study, a new integrated microsphere resonator optofluidic device is presented. The microsphere resonators are manipulated by optically-induced dielectrophoresis(ODEP) for precisely tuning the coupling distance between the resonator and a single mode rib waveguide. We fabricate single mode rib waveguide structure made by SU-8(n=1.569) on the ODEP device, with polystyrene beads(PSB, n=1.571) of 100-μm diameter were suspended in a liquid chamber with high-density sucrose solution (n=1.397). We have manipulated PSB which is pushed away and keeps a stable distance from the waveguide. Through measuring transmittance spectra at different coupling distance, PSB is operated at the over-coupling, near critical-coupling and under-coupling condition, respectively, by examining the transmission dips of the spectra. We have successfully realized a compact optofluidic platform for studying tunable microsphere optical resonators in an aqueous medium. Asalient feature of this platform is that the microsphere can be freely operated in any of the coupling conditions via a low-power image projector.