Polarization effects in optically bound particle arrays.

Sub-micron polystyrene spheres spontaneously assemble into twodimensional arrays in the evanescent field of counterpropagating laser beams at the silica-water interface. The symmetry and dynamics of these arrays depends on the particle size and the polarization of the two laser beams. Here we describe the polarization effects for particles with diameters of 390-520 nm, which are small enough to form regular 2-D arrays yet large enough to be readily observed with an optical microscope. We report the observation of rectangular arrays, three different types of hexagonal arrays and a defective array in which every third row is missing. The structure of the arrays is determined by both optical trapping and optical binding. Optical binding can overwhelm optical trapping and give rise to an array that is incommensurate with the interference fringes formed by two laser beams of the same polarization.