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Study of light-induced dipolar interactions in cold atoms assemblies

Ludovic Brossard 1
1 Laboratoire Charles Fabry / Optique Quantique
LCF - Laboratoire Charles Fabry
Abstract : Our team studies the collective behaviour of an atomic gas in the presence of dipole-dipole interactions. These interactions appear when the atoms are illuminated by a laser of wavelength lambda that is nearly resonant with an atomic transition : the atoms are polarized by the laser field, and the induced dipoles interact with each other through the field they radiate. This interaction becomes stronger when the atoms are closer to each other, and can considerably perturb the radiative behaviour of the atomic ensemble, or even prevent the simultaneous excitation of several atoms. For instance, a dense atomic cloud can behave like an optical cavity without any mirrors : the laser can excite certain radiation modes, each with its own frequency and life time, which are different from those of an individual atom. Some of these collective modes are super-radiant (the atomic cloud re-emits the stored excitation faster than an individual atom), others are sub-radiant.In order to study these phenomena, our team has built an experiment that allows the trapping of 1 up to ~500 cold rubidium atoms in a laser trap of ~1µm³ in size. We excite the atoms close to the transition at 780nm. The size of the atomic cloud, on the order of 100 nm, is close to the reduced wavelength. Also, the Doppler broadening of the atomic transition is negligible (cold atoms). The situation is therefore nearly ideal for the observation of the collective radiation modes. We observed the effects of these interactions, but no quantitative agreement with theory has been obtained so far (despite our efforts to simplify the internal atomic structure).We have thus decided to build a second version of the experimental apparatus. This challenging second version now possesses two high resolution optical axes. Not only solving some experimental problems of the previous version, it opens the road to new kind of experiments to study dipolar interactions: new regime of densities and new kind of geometries, as 1D chain of atoms for instance.
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Ludovic Brossard. Study of light-induced dipolar interactions in cold atoms assemblies. Optics [physics.optics]. Université Paris-Saclay, 2020. English. ⟨NNT : 2020UPASO002⟩. ⟨tel-02880041⟩

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