Universal trapping in a three-beam optical lattice

Abstract

We study electromagnetic trapping in an optical lattice formed by three equiangular in-plane beams. We demonstrate analytically that this optical lattice offers stable trapping sites for particles satisfying specific symmetries, irrespective of the exact nature of their electromagnetic response. Under small displacements, the particles are shown to be subject to equal restoring forces along all directions and the trap is isotropic. Though the intensity distribution of the trap forms a perfect hexagonal lattice, differences in phase variation along opposite directions cause the restoring force to be asymmetric for large displacements, resulting in a force landscape possessing only threefold symmetry. We then show numerically that this asymmetry affects the optical binding force between particles in adjacent trap positions and results in unequal shifts of their equilibrium positions. Universal trapping in the optical lattice combined with this asymmetric mechanical response of trapped particle pairs promises rich optomechanical effects.