Resonant optical propelling of microspheres: A path to selection of almost identical photonic atoms

Near field optical manipulation of microparticles is usually considered a nonresonant effect determined by the momentum conservation law. In this work we study optical propelling of polystyrene microspheres with diameters from 3 to 20 μm in the evanescent field produced by water immersed tapered microfibers. We show that the velocity of propelling increases with the average diameter of the sphere reaching extraordinary high values ~ 0.4 mm/s at modest guided optical powers (43 mW) for some of the spheres with diameters around 20 μm. We show that the optical forces exerted on such transparent dielectric spheres approach the total absorption limit and far exceed estimates for conventional nonresonant effects. By using transmission spectroscopy we demonstrate efficient coupling of light to high quality (~ 10³) whispering gallery modes in the spheres with diameters of 15-20 μm in range and argue that these effects can be responsible for resonant enhancement of the propelling velocities. We support our model by statistical analysis of a large number of propelling events for spheres with a broad range of mean diameters. This work opens the path for size sorting of spherical “photonic atoms” with almost identical resonant frequencies which can be used in various microphotonic applications.