Multidimensional optical fractionation of colloidal particles with holographic verification.

The trajectories of colloidal particles driven through a periodic potential energy landscape can become kinetically locked-in to directions dictated by the landscape's symmetries. When the landscape is realized with a structured light field, the path a given particle follows has been predicted to depend exquisitely sensitively on such properties as the particle's size and refractive index. We confirm these predictions by measuring the transport of colloidal silica spheres through arrays of holographic optical traps, using holographic video microscopy to track individual spheres' motions in three dimensions and simultaneously to measure each sphere's radius and refractive index with part-per-thousand resolution. These measurements demonstrate optical fractionation's ability to sort with part-per-thousand resolution on multiple characteristics simultaneously.