Evanescent light coupling and optical propelling of microspheres in water immersed fiber couplers

We study the propulsion of polystyrene microspheres along water immersed silica tapered fibers. We observed a nearly linear increase of the propulsion velocity with the sphere diameter increasing from 3 to 20 μm. By measuring the fiber transmission spectra we demonstrate efficient evanescent coupling of light to whispering gallery modes (WGMs) in large (>10 μm) polystyrene spheres. For 20 μm spheres we observed the depth of resonant dips ~ 3.5 dB in combination with the Q-factors ~ 103. Due to small losses in the fiber ~1-2 dB we are able to determine the power in the tapered region and to characterize quantitatively the optical propelling forces. The maximum value of the propelling velocity was 260 μm/s and was observed for 15 μm spheres with guided power of only 43 mW. Such velocities are nearly an order of magnitude higher than those observed for similar powers on waveguide structures. Using simple physical arguments we show that for spheres with diameters larger than 10 μm the experimentally observed velocities of propelling are too high to be explained by the conventional nonresonant scattering forces. We propose that these high velocities indicate that the optical forces are enhanced in such cases due to resonant coupling effects.