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Physical Review A

Stabilization of transverse solitary waves by a nonlocal response of the nonlinear medium

Dieter Suter, Tilo Blasberg

Published Dec 1, 1993• 231 citations
Publisher

Abstract

Optical pumping of atomic vapors can lead to self-focusing of resonant laser beams. The saturation of the optical pumping process at low intensity allows the study of transverse solitons in the form of self-trapped laser beams, using low-power cw lasers. The long lifetime of the optically pumped atoms allows the atomic diffusion to transport the excitation away from the interaction region. Numerical simulations show that the resulting nonlocal response of the medium represents an important mechanism that stabilizes the self-trapped beam. The theoretical predictions are confirmed by experimental results from sodium vapor.

DOI
10.1103/physreva.48.4583

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•May 1, 2013•0 citations
Optical pumping of atomic vapors can lead to self-focusing of resonant laser beams. The saturation of the optical pumping process at low intensity allows the study of transverse solitons in the form of self-trapped laser beams, using low-power cw lasers. The long lifetime of the optically pumped atoms allows the atomic diffusion to transport the excitation away from the interaction region. Numerical simulations show that the resulting nonlocal response of the medium represents an important mechanism that stabilizes the self-trapped beam. The theoretical predictions are confirmed by experimental results from sodium vapor.
Effects of radiation transport in the interaction of a laser beam with sodium vapor in an argon atmosphere are reported. Investigations of the temporal and spatial evolution of ground-state orientation, which is produced by optical pumping with a circularly polarized near-resonant laser beam, reveal a strong deviation from the expected pure diffusive behavior. Transverse profiles are narrowed considerably with increasing sodium density as well as with increasing pump intensity. The temporal decay of orientation is also drastically altered. A simplified nonlinear and nonlocal theoretical model of radiation trapping gives excellent qualitative agreement with the observations and allows an intuitive interpretation.
Summary form only given. The interaction between an intense pump beam and a weak probe beam in an atomic vapor has been the subject of numerous investigations. Besides the well-known features in the frequency spectrum of the light, interesting spatial phenomena are observed. They are due to the inhomogeneous intensity distribution of a laser beam, coupled to propagation, and range from self-focusing or defocusing effects to the generation of localized distributions of light that propagate self-similarly (bright or dark solitons). We investigate the reshaping of a probe beam due to the interaction with a pump in sodium vapor, a problem that is important in many different contexts.
There are several nonlinear optical processes that can lead to instabilities in a laser beam as it propagates through a nonlinear optical medium. Self focusing effects, for instance, can lead to the growth of new spatial frequency components in a laser beam. A laser beam can also be unstable to the growth of new spectral components. Stimulated Raman scattering is a well-known example of a process that can lead to this sort of instability. In this paper, we present the results of a series of experiments on the somewhat different instabilities that occur for the case of the propagation of a laser beam through an atomic vapor. We find that these instabilities become particularly pronounced when the frequency of the incident laser is near to that of a one- or two-photon resonance of the atom. In either case, we find that the laser beam is unstable to the growth of new frequency components, and that in general these new frequencies are not emitted collinearly with the incident laser. These instabilities are important because they place a limit on the laser intensity that can be used to drive nonlinear optical processes.
We report steady-state self-focusing, self-trapping, and self-defocusing of a cw dye laser beam in sodium vapor for frequencies within several Doppler widths of the DDD-line resonance transitions. We measured the variation of the beam profile as the light propagates through the vapor. We observed a 20-mW beam self-trapped in a 12-cm-long filament having a half-power diameter of 70 \ensuremath{\mu}m.
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.