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arXiv:1103.2478 · Physical Review A

Creation of a vortex in a Bose-Einstein condensate by superradiant scattering

Mehmet Emre Taşgın, Özgür Esat Müstecaplıoğlu, Li You

Published Dec 21, 2011• 10 citations
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Abstract

The creation of a topological vortex by a superradiant scattering of a Laguerre-Gaussian (LG) beam off an atomic Bose-Einstein condensate (BEC) is theoretically investigated. It is shown that scattered superradiant radiation can be either in a Gaussian mode without angular momentum or in a LG mode with angular momentum. The conditions leading to these two qualitatively distinct regimes of superradiance are determined in terms of the width for the pump laser and the condensate size for the limiting cases where the recoil energy is both much smaller and larger than the atomic interaction energy.

DOI
10.1103/physreva.84.063628

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The creation of a topological vortex by a superradiant scattering of a Laguerre-Gaussian (LG) beam off an atomic Bose-Einstein condensate (BEC) is theoretically investigated. It is shown that scattered superradiant radiation can be either in a Gaussian mode without angular momentum or in a LG mode with angular momentum. The conditions leading to these two qualitatively distinct regimes of superradiance are determined in terms of the width for the pump laser and the condensate size for the limiting cases where the recoil energy is both much smaller and larger than the atomic interaction energy.
We consider the scattering of an acoustic wave from a vortex induced by optical superradiance. The vortex is created by pumping a large amount of angular momentum with a Laguerre-Gaussian light beam in an atomic Bose-Einstein condensate. We derive the mean-field dynamical equations of the light-superfluid system, and obtain the equations governing the elementary excitation of the system, which result in a massless Klein-Gordon equation with source terms. This equation describes the propagation of the sound wave in an effective space-time. Employing a simplifying draining bathtub model for the vortex, we investigate the scattering of the acoustic wave in the vortex phase and obtain a condition for the acoustic superradiance. We conclude that Laguerre-Gaussian-beam-induced sudden transition from homogeneous to vortex state in the superfluid leads to a prominent observation of the acoustic superradiance.
An analysis of superradiant scattering of light by ultracold atoms is presented in which the pump and scattered light consist of hollow counterrotating Laguerre-Gaussian beams possessing orbital angular momentum. It is shown that the interaction between the Laguerre-Gaussian beams and the atoms can give rise to a superradiant instability in which orbital angular momentum is transferred to the atoms and the probe field is amplified. In the case where the atoms constitute a dilute Bose-Einstein condensate (BEC) that is trapped in a one-dimensional ring geometry, the transfer of orbital angular momentum to the BEC during the instability means that the BEC spontaneously develops rotation in the ring.
The exchange of orbital angular momentum (OAM) between paraxial optical vortex and a Bose-Einstein condensate (BEC) of atomic gases is well known. In this paper, we develop a theory for the microscopic interaction between matter and an optical vortex beyond paraxial approximation. We show how superposition of vortex states of BEC can be created with a focused optical vortex. Since the polarization or spin angular momentum (SAM) of the optical field is coupled with OAM of the field, in this case, these angular momenta can be transferred to the internal electronic and external center-of-mass motion of atoms provided both the motions are coupled. We propose a scheme for producing the superposition of matter-wave vortices using Gaussian and a focused Laguerre-Gaussian beam. We study how two-photon Rabi frequencies of stimulated Raman transitions vary with focusing angles for different combinations of OAM and SAM of optical states. We demonstrate the formation of vortex-antivortex structure and discuss interference of three vortex states in a BEC.
We present the results of an experiment on light scattering from an elongated Bose-Einstein condensate (BEC) interacting with a far-off-resonant pump laser. By collective atomic recoil lasing (CARL) a coherent superposition of two atomic wave packets with different momenta is created. Varying the intensity of a weak counterpropagating laser beam we observe the transition from the pure superradiant regime to the Bragg scattering regime, where Rabi oscillations in a two-level system are observed. The process is limited by the decoherence between the two atomic wave packets. In the superradiant regime the experiment gives evidence of a contribution to decoherence which depends on the initial velocity of the condensate. The system is described by the CARL-BEC model, which is a generalization of the Gross-Pitaevskii model to include the self-consistent evolution of the scattered field and a phase-diffusion decoherence process, which accounts for the observed damping.
We examine the use of Laguerre-Gaussian (LG) laser beams in the tripod scheme to create the coupling between spin and orbital angular momentum in a Bose–Einstein condensate (BEC) confined by a harmonic trap. We derive and solve the Gross-Pitaeskii equation numerically to obtain the density distribution of the BEC. Orbital angular momentum from the LG beam is transferred to the BEC, inducing the formation of a giant vortex at the BEC’s centre. The macroscopic flow of spinor BEC around the vortex leads to a persistent spin current in the BEC.
The non-equilibrium dynamics of the superradiant scattering of light from a BEC is experimentally investigated by monitoring the time evolution of the population in the condensate. In the experiment, a cigar-shaped Bose-Einstein condensate of /sup 87/Rb atoms is produced and is exposed to a single off-resonant laser pulse directed along the condensate symmetry axis. The superradiant back-scattering of light creates a matter wave pulse moving in the same direction of the laser beam at twice the photon recoil velocity with respect to the original condensate. The efficiency of the process is limited by decoherence between the two atomic spatially overlapped wavepackets. The temporal evolution of the condensate observed in the experiment as well as the results of a simple semiclassical model in which the condensate is described by a density operator for the motional states. The population of the two matter wavepackets as a function of the light pulse duration is reported, together with the experimental points with the analytical formula resulting from the theory of collective atomic recoil laser (CARL).