Optical separation of particles based on a dynamic interferometer

We demonstrate the sequential spatial separation of a solution consisting of a mixture of two microspheres with different diameters using a dynamic optical interferometery scheme. Two coherent lasers beams are focused together through an objective lens to form an in-plane standing wave. By linearly increasing the phase of one of incoming beams relative to the other, the optical lattice is translated. The optical forces on particles with different sizes depends on the spacing of the standing wave relative to the particle diameter; therefore, by adjusting the spacing of the standing wave so as to minimize the interaction of particles of one size with the optical lattice, all other particles can be swept out by the translating potential wells that are associated with the intensity maxima of the standing wave, while the selected particles remain trapped in the overall center of the Gaussian beam envelope of the optical lattice. Here, we demonstrate the selectivity of this optical conveyor belt by dragging smaller particles out to one side of an ensemble while simultaneously keeping the larger ones trapped. The Brownian dynamics of particles translated in an optical lattice and measurements of the associated optical force are also presented.