We have measured the optical force on isolated particles trapped in an optical lattice generated by the interference of two coherent laser beams by a method based on the equipartition theorem and by an independent method based on hydrodynamic-drag. The experimental results show that the optical force on a particle in this type of optical lattice depends <i>strongly</i> on the ratio of the particle diameter to the period of the lattice. By tuning this ratio, the force due to the optical lattice can be made to vanish. We also formed optical lattices involving two independent standing waves with different spatial periods formed by tightly focusing four laser beams which are pair wise coherent. By shifting the relative phases of the interfering beams we can advance the <i>two</i> waves in opposite directions. Depending on the spacing and the translation speed of the two interference patterns, appropriately sized particles can be translated in opposite directions; using this approach we succeeded in separating two different sizes of particles in the presence of a simulated fluid flow.
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.