guillaume Labeyrie - Université Côte d'Azur, CNRS, Institut de Physique de Nice, F-06560, Valbonne, France - Keywords: cold atoms, self-organisation, pattern formation

guillaume Labeyrie - Université Côte d'Azur, CNRS, Institut de Physique de Nice, F-06560, Valbonne, France - Keywords: cold atoms, self-organisation, pattern formation

Contribution title: Self-organization in cold atoms

We report on our observations of spontaneous pattern formation in the transverse section of a retro-reflected laser beam passing through a large cloud of cold Rubidium atoms. We have identified three distinct microscopic mechanisms for optical nonlinearity, that can under adequate experimental conditions lead to the spontaneous formation of spatial structures. The first, optomechanical mechanism is due to the spatial bunching of cold atoms under the action of the dipole force. The self-organization then takes the form of a spatial modulation of the atomic density. The second mechanism occurs when a sizable amount of the atomic population is transferred to the excited state (saturation). The observed patterns then correspond to a spatially-modulated excited state population. The third mechanism relies on optical pumping between magnetic (Zeeman) sub-states. The corresponding structures are 2D spin phases. In this last situation, a variety of magnetic-field dependent phases are observed. Some correspond to anti-ferromagnetic or ferrimagnetic arrangements, while others rely on more subtle effects such as the quantum coherence between two ground states.