Optomechanics deals with the mutual coupling of light and matter via optical forces. This can lead to spectacular examples of spontaneous spatial organization. Cold atoms are ideal for opto-mechanics since they are highly sensitive to the dipole force exerted by a spatially-inhomogeneous light field. We used a simple single pump-beam, single-mirror feedback scheme to demonstrate the spontaneous spatial organization of a cold atomic cloud in the plane transverse to the pump axis .
Cold atoms bunch into potential wells created by the dipole force in a modulated light field leading to spatial modulations of the atomic density. We identify a novel optomechanical nonlinearity resulting into the spatial structuring of the atoms.
 G. Labeyrie, E. Tesio, P. M. Gomes, G.-L. Oppo, W. J. Firth, G. R. M. Robb, A. S. Arnold, R. Kaiser and T. Ackemann, “Optomechanical self-structuring in a cold atomic gas“, Nat. Photon. 8, 321–325 (2014)