Spectrally resolved resonant propulsion of dielectric microspheres

Use of resonant light forces opens up a unique approach to high‐volume sorting of microspherical resonators with much higher uniformity of resonances compared to that in coupled‐cavity structures obtained by the best semiconductor technologies. In this work, the spectral response of the propulsion forces exerted on polystyrene microspheres near tapered microfibers is directly observed. The measurements are based on the control of the detuning between the tunable laser and internal resonances in each sphere with accuracy higher than the width of the resonances. The measured spectral shape of the propulsion forces correlates well with the whispering‐gallery mode resonances in the microspheres. The existence of a stable radial trap for the microspheres propelled along the taper is demonstrated. The giant force peaks observed for 20‐μm spheres are found to be in a good agreement with a model calculation demonstrating an efficient use of the light momentum for propelling the microspheres. The optical propulsion of polystyrene microspheres is studied in tapered microfiber couplers in both spectral and time domains simultaneously. By tuning the wavelength of the guided mode, it is shown that the spectral shape, position and magnitude of the observed propelling force peaks are explained by efficient light‐momentum transfer to microspheres under resonance with their whispering‐gallery modes. The stable radial trap for the moving microspheres is demonstrated.