Bimodal momentum distribution of laser-cooled atoms in optical lattices

We study, numerically and experimentally, the momentum distribution of atoms cooled in optical lattices. Using semi-classical simulations, we show that this distribution is bimodal, made up of a central feature corresponding to "cold", trapped atoms, with tails of "hot", untrapped atoms, and that this holds true also for very shallow potentials. Careful analysis of the distribution of high-momentum untrapped atoms, both from simulations and experiments, shows that the tails of the distribution does not follow a normal law, hinting at a power-law distribution and non-ergodic behavior. We also revisit the phenomenon of d\'ecrochage, the potential depth below which the temperature of the atoms starts increasing.