Strong local-field effect on dynamics of a dilute atomic cloud irradiated by two counterpropagating optical fields: beyond standard optical lattices*

We study a recent experiment (K. Li et al., Phys. Rev. Lett. 101, 250401 (2008)) on diffracting a Bose-Einstein condensate by two counterpropagating optical fields. Including the local field effect, we explain asymmetric momentum distribution and self-imaging of the BEC in a self-consistent way, and find that the self-imaging is not dependent on the intensity difference of the two optical fields, but on the light-condensate interaction time. We show further that the local field effect leads to deformation of an optical lattice, and thus is essential for getting better quantitative analysis of other current optical lattice experiments of cold atoms. Moreover, intensity imbalance of the two optical fields could be applied as a new means to tailor both cold atom dynamics and optical propagation.