  A two-dimensional periodic optical force field, which combines conservative
dipolar forces with vortices from radiation pressure, is proposed in order to
influence the diffusion properties of optically susceptible nano-particles. The
different deterministic flow patterns are identified. In the low noise limit,
the diffusion coefficient is computed from a Mean First Passage Time (MFPT) and
the Most Probable Escape Paths (MPEP) are identified for those flow patterns
which possess an stable stationary point. Numerical simulations of the
associated Langevin equations show remarkable agreement with the analytically
deduced expressions. Modifications of the force field are proposed so that a
wider range of phenomena could be tested.
