Polarization and particle size dependence of radiative forces on small metallic particles in evanescent optical fields. Evidences for either repulsive or attractive gradient forces.

We have observed the motion of metallic particles above various optical waveguides injected by 1064nm radiation. Small gold particles (250nm diameter) are attracted towards the waveguide where the intensity of the optical field is maximum, and are propelled at high velocity (up to 350mum/s) along the waveguide due to radiation pressure. The behaviour of larger metallic particles (diameter >600nm) depends on the polarization of the evanescent field: for TM polarization they are attracted above the waveguide and propelled by the radiation pressure; for TE polarization they are expelled on the side of the waveguide and propelled at much smaller velocity. This is consistent with calculations of radiative forces on metallic particles by Nieto-Vesperinas et al. 3D-finite element method calculations carried out for our experimental situations confirm the observed dependence with the polarization of the field and the size of the particles. These observations open the way to the development of new microsystems for particles manipulations and sorting applications.