Optical waveguide trapping forces on hollow glass spheres

Three dimensional finite element method is employed to determine optical trapping forces on hollow glass spheres on an optical waveguide. The evanescent field from the waveguide interacts mostly with the shell of the hollow glass sphere. We describe how the optical forces vary with shell thickness and particle size, and find the minimum shell thickness allowing trapping and propulsion of hollow glass spheres. Hollow glass spheres with shell thickness less than the minimum are repelled away from the optical waveguide. The simulation results are compared with analytical Mie calculations and experimental data.