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-’s, this kind of force could be deployed for size-selective manipulation with a very high accuracy , as well as simultaneous particle-sorting according to their size or resonant frequency.
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The fact that Eq. (1) is a function of is merely a consequence of choosing different coordinate origins for particles with different sizes, i.e., the origin is from the interface where the intensity is fixed.
The optical force is a quadratic function of the fields. At resonance, the WGM’s field is dominating over the incident field. The force induced solely by the incident field is negligibly small, whereas the force induced solely by the WGM’s field is identically zero owing to symmetry. As a result, is a bilinear product of the incident driving field and the WGM’s field.
