Optomechanics deals with the mutual coupling of light and matter via optical forces. This can lead to spectacular examples of spontaneous spatial organisation. Cold atoms and are ideal for optomechanics since they are highly sensitive to the dipole force exerted by a spatially-inhomogeneous light field. In collaboration with EQOP and the Cold Atom Group at INPHYNI, Nice, we used a simple single pump-beam, single-mirror feedback scheme to demonstrate the spontaneous optomechancal organization of a cold atomic cloud in the plane transverse to the pump axis [1]. 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 in the simultaneous spatial structuring of the atomic density and optical intensity. This nonlinearity is also predicted to occur in Bose-Einstein Condensates [2], and can produce rotating optical and atomic density patterns when the pump beam possesses orbital angular momentum (OAM) [3].

Optomechanical self-organisation in a cold gas of Rb atoms [1].	Spontaneous formation of rotating ring lattices of light (left) and atoms (right) [3]

[1] 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).

[2] G. R. M. Robb, E. Tesio, G.-L. Oppo, W. J. Firth, T. Ackemann, and R. Bonifacio . “Quantum Threshold for Optomechanical Self-Structuring in a Bose-Einstein Condensate”, Phys. Rev. Lett. 114, 173903 (2015).

[3] Giuseppe Baio, Gordon R. M. Robb, Alison M. Yao, and Gian-Luca Oppo, “Optomechanical transport of cold atoms induced by structured light”, Phys. Rev. Research 2, 023126 (2020).

Funders: Leverhulme Trust & EU H2020 (ColOpt)