Long-range density patterns in a six-beam optical lattice from polarization interference

Abstract

We report a correlation between periodic patterns of cold-atom density and field polarization that gives new insight into a nearly 30 year old question about the role of optical phases in laser cooling. The laser field of three intersecting pairs of counter-propagating σ + − σ − beams is widely used for cooling of atoms in magneto-optical traps and optical molasses. This six-beam optical lattice differs from four-beam lattices in having two “excess” phase constants of the optical standing waves. These relative time phases are independent degrees of freedom that affect key lattice properties, such as the polarization of the potential wells and the sub-Doppler cooling process. In this work, we create one-dimensional and two-dimensional spatial gradients of the optical phases in a six-beam lattice by slightly tilting the retro-reflected beams. This tilt imparts a long-range periodicity ( ∼ m m ) to the lattice polarization, creating a superlattice in which atoms are seen to diffuse into a periodic density distribution over several milliseconds. We find that high density is correlated with low field ellipticity; atoms move preferentially into regions where the field is linearly polarized. Such a non-uniform density may be important in cold collision experiments and may have applications in the preparation of atoms for atomic clocks and quantum computation.