Trapping colloidal dielectric microparticles with overlapping evanescent optical waves

We experimentally demonstrate the creation of a stable surface trap for colloidal microparticles in a high-intensity evanescent optical field that is produced by total internal reflection of two counter-propagating and mutually incoherent laser beams. While the particles confined in the trap undergo fast Brownian motion, they never "stick" to the surface - not even at high optical powers - but rather levitate above the surface. If many particles are stored in the trap, they tend to form a well ordered self-organized array. We apply a numerical model based on the general energy-momentum tensor formalism to evaluate the overall optical force acting on a trapped particle. The optical-field parameters are calculated using the finite element method. The simulations show that for small particles a sharp repulsive potential at the surface - required for the levitation - can have neither optical nor light-induced thermal origin. Among the possible non-optical forces, electrostatic double-layer repulsion is often considered to be the origin of the levitation. We find, however, that the experimentally observed levitation of small particles in a high-intensity evanescent-wave trap cannot be explained by this effect. (C) 2012 Elsevier B.V. All rights reserved.