Force of surface plasmon-coupled evanescent fields on Mie particles

Forces acting on Mie particles by surface plasmon-coupled evanescent fields are theoretically analyzed. An expression of the evanescent field produced in Kretschmann (or Sarid) geometry is derived in terms of vector spherical wave functions, while multiple reflections between the Mie particle and the metal boundary are taken into account. The Cartesian components of the force on a Mie particle by the evanescent field are analytically formulated and numerically evaluated. We show that forces are increased by one or two orders at metal boundaries than at dielectric boundaries. For a glass bead, the force normal to the metal boundary is strong compared with the tangential force. For a silver-coated glass bead, the tangential force is greater than (or comparable with) the normal force. The strong tangential force can be useful for rotating submicrometer-sized rotors at a metal boundary and the normal force can keep the rotors close to the metal boundary.