Self-organization of a laser-driven cold gas in a ring cavity

Abstract

Abstract. -A gas of linearly polarizable particles transversely pumped by an off-resonant laser and interacting with the counterpropagating radiation modes of a ring cavity is studied. Depending on the pump intensity and the detunings the gas can form a self-organized density grating that enhances the feeding of the cavity. We investigate the system via a mean-field approach and find the thermodynamic phases of i) uniform distribution, ii) self-organized Bragg lattice, iii) lattice with defects, and iv) instability. The occurrence of these phases as a function of the pump intensity and particle density is fully mapped.The physics of ultracold atoms has been a proliferating field these years. There is a renewed interest in fundamental many-body phenomena which can be investigated in well-controlled, weakly interacting atomic ensembles. Phase transition into a Bose-Einstein condensate is being routinely realized with alkali atoms, and various other manifestations of quantum statistics, superfluidity, etc., are observed. The system of cold atoms is particularly attractive because the collisional properties are partially tunable.The nature of atom-atom interaction is a central issue in the understanding of many-body effects. This interaction is dominated by the dipole-dipole coupling in a dense cloud of cold atoms illuminated by quasi-resonant laser fields. On the one hand, this amounts to a measurable modification of the optical properties of the cloud, e.g., a nonlinear density dependence of the refractive index [1], or slow diffusion of light [2]. On the other hand, it induces interatomic forces, which eventually cause an instability of the homogeneous distribution of the cloud [3].In this letter we analyze a system presenting a strong interplay of the electromagnetic field generating a collective behaviour of an ensemble of polarizable particles, and the backreaction of those particles on the dynamics of the field. The system is shown to produce a phase transition between the homogeneous spatial distribution and a regular pattern bound by the electromagnetic field. This effect can be described in terms of analytical expressions accounting for the density of the particles and laser pumping strength by virtue of the simple geometry.c EDP Sciences Article published by EDP Sciences and available at http://www.edpsciences.org/epl or http://dx

Observation of Optomechanical Strain in a Cold Atomic Cloud

“…Interactions are also tuned by optical Feshbach resonance [17], optical cavities [18], or radio frequency Feshbach resonance [19]. Interactions can be induced by shining a laser beam on the atoms and creating a feedback mechanism to their response by an externally pumped cavity or a half cavity [20][21][22][23][24][25]. The electrostriction force reported here is a new kind of induced force between atoms, and may be useful in cold atoms and quantum degenerate atom experiments.…”

Atom-light crystallization of Bose-Einstein condensates in multimode cavities: Nonequilibrium classical and quantum phase transitions, emergent lattices, supersolidity, and frustration

“…Exploring precisely such interactions has been the central theme of cavity QED [7]. Traditionally, cavity QED has aimed to realize systems involving a single atom coupled to a single mode of the electromagnetic field; however, the physics of many atoms coupled to one (or more) electromagnetic modes, i.e., many-body cavity QED, has also been studied extensively in recent work [8][9][10][11][12][13][14][15][16]. In particular, it was predicted in in Refs.…”

“…With multimode cavities, by contrast, self-organization results in the breaking of continuous symmetries, both in the collective choice of which mode(s) to populate with photons and in the choice of relative phases between the modes. For example, in the case of the ring cavity (which consists of two counter-propagating traveling-wave modes [11]), the atoms must collectively choose the (continuous) relative phase between the two counter-propagating modes, thus setting the location of the antinodes of the cavity field. As with real solid-state crystallization, the breaking of this continuous symmetry induces rigidity with respect to lattice deformations.…”

Self ordering threshold and superradiant backscattering to slow a fast gas beam in a ring cavity with counter propagating pump

“…As a consequence of this acceleration, R − will be time dependent in general. Hence in contrast to atomic selforganization in a standing wave cavity field [24], we cannot expect the system to reach a time independent self consistent atomfield steady state. Nevertheless, as we will see below, quasistationary self consistent atom field distributions can still form, if this acceleration is not too fast.…”