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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Spontaneous opto-mechanical structures in cold atomic gases

Enrico Tesio, Gordon R. M. Robb, T. Ackemann, P. M. Gomes, Aidan S. Arnold, W. J. Firth, Gian‐Luca Oppo, G. Labeyrie, Robin Kaiser

Published May 1, 2013• 0 citations
Publisher

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 pattern-forming instabilities in hot gases where optical nonlinearities arise from the internal structure of the atoms and spatio-temporal structures are encoded in the populations and coherences of the medium. Dipole forces acting on the center-of-mass of laser-cooled 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 pattern-forming instability. We stress that the resulting spatial structures are encoded also in the spatial density distribution, effectively leading to the self-assembly of an optical atomic lattice.

DOI
10.1109/cleoe-iqec.2013.6801810

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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 pattern-forming instabilities in hot gases where optical nonlinearities arise from the internal structure of the atoms and spatio-temporal structures are encoded in the populations and coherences of the medium. Dipole forces acting on the center-of-mass of laser-cooled 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 pattern-forming instability. We stress that the resulting spatial structures are encoded also in the spatial density distribution, effectively leading to the self-assembly of an optical atomic lattice.
Summary 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 self-organization of atomic ensembles due to opto-mechanical coupling has received a lot of interest in recent years [3].We report on the observation of transverse self-organization 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 retro-reflected to the atoms by a high-reflectivity 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 μs 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 μs), high pump intensity and cloud optical density, the instability relies on the Kerr effect (electronic nonlinearity). For longer pump durations (- 100 μs), the instability is driven by the opto-mechanical 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,ternal degrees of freedom.
The rapidly developing field of optomechanics aims at the combined control of optical and mechanical (solid-state or atomic) modes. In particular, laser cooled atoms have been used to exploit optomechanical coupling for self-organization 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 opto-mechanical self-assembly with adjustable length scales and can be potentially extended to quantum degenerate gases.
Self-structuring via symmetry breaking is a prominent collective phenomenon occurring in cold and ultracold atoms coupled to optical fields [1] . Experimental realizations by means of trapped dipolar gases and optical cavities have shed light onto supersolidity and crystallization in out-of-equilibrium atomic systems [2] , [3] . Recent work demonstrated the occurrence of structural phase transitions between hexagonally coordinated supersolid phases in dipolar condensates [4] . In this work, we address 2D structural transitions occurring in an ensemble of cold thermal atoms with effective optomechanical interactions mediated by a coherent beam retro-reflected by means of a feedback mirror. The feedback loop scheme is represented in Fig. 1(a) , where the dipole potential induces atomic density structures which, in turn, scatter photons into side-band modes enhancing the potential itself and leading to transverse light-atom self-structuring, sharing similarities with an effective-Kerr nonlinear medium [5] .
We report a new nonlinear optical process that occurs in a cloud of cold atoms at low-light-levels when the incident optical fields simultaneously polarize, cool, and spatially-organize the atoms. We observe an extremely large effective fifth-order nonlinear susceptibility of χ(⁵) = 7.6 × 10⁻¹⁵ (m/V)⁴, which results in efficient Bragg scattering via six-wave mixing, slow group velocities (∼ c/10⁵), and enhanced atomic coherence times (> 100 μs). In addition, this process is particularly sensitive to the atomic temperatures, and provides a new tool for in-situ monitoring of the atomic momentum distribution in an optical lattice. For sufficiently large light-matter couplings, we observe an optical instability for intensities as low as ∼ 1 mW/cm² in which new, intense beams of light are generated and result in the formation of controllable transverse optical patterns.