Size-selective optical forces for microspheres using evanescent wave excitation of whispering gallery modes

TL;DR

This paper demonstrates that evanescent wave illumination can generate size-selective optical forces on microspheres by exploiting whispering gallery mode resonances, enabling precise manipulation and sorting based on particle size or resonant frequency.

Contribution

It introduces a method to achieve highly size-selective optical forces using evanescent wave excitation of whispering gallery modes, with potential for high-accuracy particle sorting.

Findings

Optical forces differ significantly on- and off-resonance.

Size-selective manipulation can be achieved with high precision (~1/Q).

Potential for particle sorting based on size or resonant frequency.

Abstract

We show that when a microsphere is illuminated by an evanescent wave, the optical forces on- and off- whispering gallery mode (WGM) resonance can differ by several orders of magnitude. Such size selective force allows one to selectively manipulate the resonating particles, while leaving those particles at off-resonance untouched. As WGM resonances have very high-Q's, this kind of force could be deployed for size-selective manipulation with a very high accuracy (~1/Q), as well as simultaneous particle-sorting according to their size or resonant frequency.