Publications (125)286.29 Total impact

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ABSTRACT: We present an experimental and theoretical investigation of spontaneous pattern formation in the transverse section of a single retroreflected laser beam passing through a cloud of cold Rubidium atoms. In contrast to previously investigated systems, the nonlinearity at work here is that of a 2level atom, which realizes the paradigmatic situation considered in many theoretical studies of optical pattern formation. In particular, we are able to observe the disappearance of the patterns at high intensity due to the intrinsic saturable character of 2level atomic transitions.  [Show abstract] [Hide abstract]
ABSTRACT: Theoretical analysis of the optomechanics of degenerate bosonic atoms with a single feedback mirror shows that selfstructuring occurs only above an input threshold that is quantum mechanical in origin. This threshold also implies a lower limit to the size (period) of patterns that can be produced in a condensate for a given pump intensity. These thresholds are interpreted as due to the quantum rigidity of BoseEinstein condensates, which has no classical counterpart. Above the threshold, the condensate selforganizes into an ordered supersolid state with a spatial period selfselected by optical diffraction.  [Show abstract] [Hide abstract]
ABSTRACT: We consider Bloch oscillations of ultracold atoms stored in a onedimensional vertical optical lattice and simultaneously interacting with a unidirectionally pumped optical ring cavity whose vertical arm is collinear with the optical lattice. We find that the feedback provided by the cavity field on the atomic motion synchronizes Bloch oscillations via a modelocking mechanism, steering the atoms to the lowest Bloch band. It also stabilizes Bloch oscillations against noise, and even suppresses dephasing due to atomatom interactions. Furthermore, it generates periodic bursts of light emitted into the counterpropagating cavity mode, providing a nondestructive monitor of the atomic dynamics. All these features may be crucial for future improvements of the design of atomic gravimeters based on recording Bloch oscillations. 
Article: Chaotic resonances of a BoseEinstein condensate in a cavity pumped by a modulated optical field
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ABSTRACT: We present a theoretical analysis of a BoseEinstein condensate (BEC) enclosed in an optical cavity driven by a modulated external laser beam where the cavity field variable is adiabatically eliminated. The modulation of the amplitude of the pump laser induces nonlinear resonances and the widespread presence of chaotic oscillations even when repulsive atomatom interactions are negligible. Close to resonance, varying the modulation amplitude by just a few percent causes abrupt and erratic changes to the output laser intensity with peak power increasing by almost an order of magnitude. We also use a simplified model of the BECcavity system that considers only a small number of spatial modes of the BEC to show that, despite the disruptive presence of a modulation in the pump beam, the system can still be considered to be lowdimensional.  [Show abstract] [Hide abstract]
ABSTRACT: We study nonequilibrium spatial selforganization in cold atomic gases, where longrange spatial order spontaneously emerges from fluctuations in the plane transverse to the propagation axis of a single optical beam. The selforganization process can be interpreted as a synchronization transition in a fully connected network of fictitious oscillators, and described in terms of the Kuramoto model. 
Conference Paper: Nonlinear Optomechanical Patterns and Dissipative Solitons
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ABSTRACT: Experimental, theoretical and numerical evidence of optomechanic selfstructuring of a laser beam in a cloud of cold atoms in a singlemirror feedback configuration is presented. Optomechanic dissipative solitons can be encoded in the atomic density. 
Conference Paper: Conservative nonlinear interaction of light and a BoseEinstein condensate in a lossy optical cavity
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ABSTRACT: We consider a BoseEinstein condensate in an optical cavity being driven by an external laser beam. The modulated optical lattice induces conservative chaotic oscillations that are ubiquitous for pump rates that exceed a critical value.  [Show abstract] [Hide abstract]
ABSTRACT: We present a new technique for stabilizing and monitoring Bloch oscillations of ultracold atoms in an optical lattice under the action of a constant external force. In the proposed scheme, the atoms also interact with a unidirectionally pumped optical ring cavity whose one arm is collinear with the optical lattice. For weak collective coupling, Bloch oscillations dominate over the collective atomic recoil lasing instability and develop a synchronized regime in which the atoms periodically exchange momentum with the cavity field.  [Show abstract] [Hide abstract]
ABSTRACT: We present a theoretical analysis of a BoseEinstein condensate (BEC) enclosed in an optical cavity driven by an external laser beam. In the limit where the cavityfield dynamics is adiabatically eliminated, the interaction between the condensate and the cavity displays a range of nonlinear dynamical behaviors such as multistability, quasiperiodicity, and conservative chaotic oscillations. We show that chaotic oscillations are ubiquitous when repulsive atomatom interactions are included in the model. Despite the complexity of the full coupled BECcavity system, the essential features of its nonlinear and chaotic behavior are lowdimensional and are well described by considering the evolution of only a few motional states of the BEC.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate transverse symmetrybreaking instabilities emerging from the optomechanical coupling between light and the translational degrees of freedom of a collisionless, dampingfree gas of cold, twolevel atoms. We develop a kinetic theory that can also be mapped on to the case of an electron plasma under ponderomotive forces. A general criterion for the existence and spatial scale of transverse instabilities is identified; in particular, we demonstrate that monotonically decreasing velocity distribution functions are always unstable. 
Conference Paper: Optomechanical selforganization in cold atomic gases
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ABSTRACT: We discuss the formation of optomechanical structures from the interaction between linear dielectric scatterers and a light field via dipole forces without the need for optical nonlinearities. The experiment uses a high density sample of Rb atoms in a single mirror feedback geometry. We observe hexagonal structures in the light field and a complementary honeycomb pattern in the atomic density. Different theoretical approaches are discussed assuming either viscous damping of the atomic velocity or not. The interplay between electronic and optomechanical nonlinearities is analyzed. A prediction for dissipative light  matter density solitons is given. The investigations demonstrate novel prospects for the manipulation of matter in a pattern forming system in which quantum effects should be accessible.  [Show abstract] [Hide abstract]
ABSTRACT: We reply to the Comment of Petrillo et al. and show that the main result of our paper is unchanged.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the coupled dynamics of light and cold atoms in a unidirectional ring cavity, in the regime of low saturation and linear singleatom response. As the dispersive optomechanical coupling between light and the motional degrees of freedom of the atoms makes the dynamics nonlinear, we find that localized, nonlinearitysustained and bistable structures can be encoded in the atomic density by means of appropriate control beams.  [Show abstract] [Hide abstract]
ABSTRACT: The rapidly developing field of optomechanics aims at the combined control of optical and mechanical (solidstate or atomic) modes. In particular, laser cooled atoms have been used to exploit optomechanical coupling for selforganization in a variety of schemes where the accessible length scales are constrained by a combination of pump modes and those associated to a second imposed axis, typically a cavity axis. Here, we consider a system with many spatial degrees of freedom around a single distinguished axis, in which two symmetries  rotations and translations in the plane orthogonal to the pump axis  are spontaneously broken. We observe the simultaneous spatial structuring of the density of a cold atomic cloud and an optical pump beam. The resulting patterns have hexagonal symmetry. The experiment demonstrates the manipulation of matter by optomechanical selfassembly with adjustable length scales and can be potentially extended to quantum degenerate gases. 
Conference Paper: Hexagonal selfstructuring due to optomechanical nonlinearities in cold atomic gases
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ABSTRACT: We demonstrate an optomechanical instability in a sample of cold atoms driven by a single laser beam in presence of a feedback mirror. Hexagonal light filaments propagate in atomdepleted tubes forming a honeycomb lattice. 
Conference Paper: Chaotic Dynamics of BoseEinstein Condensates in Optical Cavities
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ABSTRACT: form only given. We perform a theoretical and numerical analysis of a system comprising a BoseEinstein condensate (BEC) interacting with a laser beam in an optical cavity. Recent studies have modelled this system with two motional modes coupled with a light mode and have shown nonlinear behaviour, e.g. bistability [1,2]. Our approach tackles the full GrossPitaevskii equation coupled to the cavity field as well as expansions in a number of motional modes.We consider a fixed value of the cavitypump detuning and increase the pump intensity to cross the region of bistability (see Fig. 1 left panel). The system then displays a variety of behaviours, including regular oscillations, chaotic dynamics, and quasiperiodic behaviour, as displayed in the right panel of Fig. 1. In Fig. 1 we have used a coupledequation system comprising the SchrÃ¶dinger wave equation and a cavityfield evolution [3], where we have adiabatically eliminated the latter leading to a conservative dynamics. The chaotic dynamics here is not induced by the detector noise and is an intrinsic property of the lightBEC interaction.Our second model considers a multiplemodes expansion. By using just three modes, the chaotic dynamics observed in the full simulation is recovered. The bifurcation structure, typical of conservative dynamics in the vicinity of a separatrix, is identified through the use of projections and power spectra. Our results suggest that chaotic oscillations in this system are deterministic in nature and arise purely from nonlinear effects. In more general terms, the model is a novel example of quantum matter exhibiting chaotic behaviour and invites further research.  [Show abstract] [Hide abstract]
ABSTRACT: We study an atomcavity system in which the cavity has several degenerate transverse modes. Moderesolved cavity transmission spectroscopy reveals wellresolved atomcavity resonances for several cavity modes, a signature of collective strong coupling for the different modes. Furthermore, the experiment shows that the cavity modes are coupled via the atomic ensemble contained in the cavity. The experimental observations are supported by a detailed theoretical analysis. The work paves the way to the use of interacting degenerate modes in cavitybased quantum information processing, where qubits corresponding to different cavity modes interact via an atom shared by the two modes. Our results are also relevant to the experimental realization of quantum spin glasses with ultracold atoms. 
Conference Paper: Spontaneous OptoMechanical Structures in Cold Atomic Gases
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ABSTRACT: We theoretically, numerically and experimentally investigate spontaneous transverse instabilities in cold atomic gases, arising from the action of dispersive light forces. Previous research focused on patternforming instabilities in hot gases where optical nonlinearities arise from the internal structure of the atoms and spatiotemporal structures are encoded in the populations and coherences of the medium. Dipole forces acting on the centerofmass of lasercooled atoms, being dependent on gradients of the optical intensity, are also nonlinear in nature: previous studies focused, for instance, on beam filamentation. Here we investigate the situation where a positive feedback loop is present in the system leading to a patternforming instability. We stress that the resulting spatial structures are encoded also in the spatial density distribution, effectively leading to the selfassembly of an optical atomic lattice.  [Show abstract] [Hide abstract]
ABSTRACT: form only given. Spontaneous optical pattern formation occurs in a variety of nonlinear systems [1], including hot atomic vapors [2]. On the other hand, the spatial selforganization of atomic ensembles due to optomechanical coupling has received a lot of interest in recent years [3].We report on the observation of transverse selforganization of a cold atomic cloud (issued from a magnetooptical trap) under the action of a single pump laser beam. Two symmetries (translation and rotation) in the plane orthogonal to the beam propagation direction are spontaneously broken. We use a simple optical feedback scheme [4], where the transmitted pump beam is retroreflected to the atoms by a highreflectivity mirror located at a distance d behind the cloud. This feedback loop transforms phase fluctuations of the transmitted wave into intensity fluctuations, which then react on the atomic medium. If the feedback is positive, a transverse instability can develop leading to the spontaneous apparition of patterns in the transmitted pump intensity profile, as shown in the figure below (left).Using a weak probe beam sent a few tens of Î¼s after the e,tinction of the pump, we demonstrate that the instability also results in a transverse spatial ordering of the atomic medium as shown in the right image. The cold atoms thus e,perience strong spatial bunching due to the dipole force associated to the inhomogeneous intensity distribution. We identified two different instability regimes. For short pump durations ( 1 Î¼s), high pump intensity and cloud optical density, the instability relies on the Kerr effect (electronic nonlinearity). For longer pump durations ( 100 Î¼s), the instability is driven by the optomechanical effect, resulting in lower intensity and optical density thresholds than for the electronic nonlinearity. These observations are well reproduced by a theoretical model including the coupled dynamics of the light field and the atomic e,ter al degrees of freedom.
Publication Stats
1k  Citations  
286.29  Total Impact Points  
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Institutions

19942015

University of Strathclyde
 Department of Physics
Glasgow, Scotland, United Kingdom


1996

Russian Academy of Sciences
Moskva, Moscow, Russia
