Tuning the optical forces on- and off-resonance in microspherical photonics

Light pressure effect has been discovered long ago and has been used as an optical method to manipulate micro- and nanoparticles. It is usually considered as a nonresonant effect determined by the transfer of the momentum of light. However, recently we have observed that large polystyrene microspheres of 15 - 20 μm diameters supporting high quality whispering gallery resonances can be optically propelled in water at an extraordinary high velocity along tapered fibers under resonant conditions. In this work we compare on- and off-resonant optical forces in microspherical photonics by controlling the detuning between the laser emission line and whispering gallery resonances. Our approach involves manipulation with microspheres using conventional optical tweezers and their advanced spectroscopic characterization in fiber-integrated setups. We demonstrate dramatic difference in the optical forces exerted on microspheres in the on-resonant and off-resonant cases. This method can be used to study spectral properties of the resonantly enhanced forces in microspherical photonics.