Waveguide trapping of hollow glass spheres

Microparticles can be trapped and propelled by the evanescent field of optical waveguides. As the evanescent field only stretches 100-200?nm from the surface of the waveguide, only the lower caps of the microparticles interact directly with the field. This is taken advantage of by trapping hollow glass spheres on waveguides in the same way as solid glass spheres. For the chosen waveguide, numerical simulations show that hollow microspheres with a shell thickness above 60?nm can be stably trapped, while spheres with thinner shells are repelled. The average refractive index of the sphere-field intersection volume is used to explain the result in a qualitative way.