Multiple Self-Organized Phases and Spatial Solitons in Cold Atoms Mediated by Optical Feedback

Controlling mechanical states of atoms is one of the ultimate and most impressive applications that has arisen from the study of light-matter interaction at the nanoscale.  This is made possible by the fact that photons exert forces onto atoms, capable of cooling and trapping them into geometric configurations that allow us to reproduce the complex world of solid-state physics. Those ideas trace back to the pioneering works of Ashkin on optical tweezing, that led to the Nobel prize for Physics shared with Morou and Strickland in 2018.

A Strathclyde-based team of researchers within the European training network ColOpt has recently investigated a configuration where an atomic cloud is placed in front of a mirror reflecting a tailored driving laser beam, a system invented by Willie Firth (see Figure a). Theoretical studies developed by Giuseppe Baio, Gordon Robb, Alison Yao, Gian-Luca Oppo from the CNQO group and Thorsten Ackemann from the EQOP group, have revealed novel transitions, induced by opto-mechanical forces, between structurally different light/atom-density phases. Those phases appear spontaneously by means of self-organisation and are characterized by hexagonal, stripe, honeycomb ordering (see Figure b). Simulating the complex physics of the atomic cloud, supported by a rigorous analytic framework, has shown an interesting and unexpected behaviour beyond the Kerr approximation, previously introduced by Ashkin himself. As a by-product, a novel kind of solitary excitation (spatial soliton), characterized by an atom-density peak with particle-like properties, can be transported with an input beam possessing a particular vortex structure (see Figure c).

Those results provide new insight into the physics of structural transitions and self-organisation in cold and ultracold atomic gases, and similar spontaneous symmetry breaking phenomena in photonics systems.

The research has been published in Physical Review Letters on May 21st, 2021: G. Baio, G. R. M. Robb, A. M. Yao, G.-L. Oppo, and T. Ackemann, Multiple Self-Organized Phases and Spatial Solitons in Cold Atoms Mediated by Optical Feedback, Phys. Rev. Lett. 126, 203201, 2021.

June 2021