Spontaneous opto-mechanical structures in cold atomic gases

We theoretically, numerically and experimentally investigate spontaneous transverse instabilities in cold atomic gases, arising from the action of dispersive light forces. Previous research focused on pattern-forming instabilities in hot gases where optical nonlinearities arise from the internal structure of the atoms and spatio-temporal structures are encoded in the populations and coherences of the medium. Dipole forces acting on the center-of-mass of laser-cooled atoms, being dependent on gradients of the optical intensity, are also nonlinear in nature: previous studies focused, for instance, on beam filamentation. Here we investigate the situation where a positive feedback loop is present in the system leading to a pattern-forming instability. We stress that the resulting spatial structures are encoded also in the spatial density distribution, effectively leading to the self-assembly of an optical atomic lattice.