Spectrally resolved resonant propulsion of dielectric microspheres

TL;DR

This paper demonstrates spectrally resolved resonant propulsion of dielectric microspheres, showing how precise laser detuning can control forces and trap microspheres efficiently, with potential for high-volume sorting applications.

Contribution

It provides the first direct measurement of spectral propulsion forces on microspheres near tapered microfibers, correlating force spectra with whispering-gallery modes and demonstrating stable trapping.

Findings

Spectral propulsion forces match whispering-gallery mode resonances.

Stable radial trapping of microspheres along tapered fibers.

Large force peaks enable efficient microsphere propulsion.

Abstract

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-{\mu}m 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.