Optical Manipulation Of Gold Micro And Nano-Particles On Silicon Nitride Waveguides: Impact Of Polarization And Particle Size On Gradient Forces

Kawata and Tani's [4] experiments showed that the evanescent field created on the surface of an ion exchanged waveguide could trap and move microparticles. This opened up the possibility of combining conventional optical trapping with integrated optics in order to create new microsystems for the manipulation of particles or biological objects. Recently, the use of strip silicon nitride waveguides increased the performances of these systems enabling higher particles speeds and reduced guided power [12].Our experiments demonstrate that polarization affects drastically the way particles are propelled along the waveguide surface. For example in TM polarization, 0,6 mu m diameter gold particles are moving along the center of the waveguide whereas in TE, they are propelled along its sides. Moreover, it appears that gradient forces involved in this phenomenon depend on the particle size.To understand this behavior, a numerical approach of the problem based on the finite element method has been developed. This method enables the calculation of the 3D distribution of the electric fields. The resulting optical forces are calculated thanks to the Maxwell stress tensor formalism.This first experimental and theoretical illustration of repulsive gradient forces on metallic particles opens up perspectives for polarization based sorting systems.