Resumne: 

e have been studying large (mm) scale periodic density variations appearing in a conventional 6-beam magneto-optic trap, related to the optical lattice created by the 6 laser beams. This phenomenon was first reported 30 years ago but accurate theoretical models have been difficult to implement due to the complexity of the optical fields and the atomic hyperfine manifolds involved.  Proposed explanations for the density patterns include variation in optical light-shift potentials, friction forces from polarization-gradient cooling, and optical vortices, but no clear answer has emerged.  All of these effects depend upon the two relative optical time phases between the 3 beam-pairs and these phases depend on the path lengths taken by the laser light in reaching the interaction region.  Thus a small misalignment angle deliberately introduced in one beam of a nearly counter-propagating pair produces a gradual phase change across the interaction region.  This in turn gives rise to long scale variations in the optical field – a superlattice imposed on the optical lattice. 

     In this talk we report studies of 2D density patterns that indicate for the first time which optical phases induce high density.  Atoms accumulate in regions where the lattice is primarily linearly polarized, which is when the relative optical phases are zero.  The atomic motion leading to accumulation is due to diffusion of atoms as they jump between optical potential wells, so our density distribution methods and related time-of-formation measurements may give insight into that diffusion process.