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

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 (similar to 1/Q), as well as simultaneous particle-sorting according to their size or resonant frequency. (C) 2008 American Institute of Physics.