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Instabilités crées par mélange à quatre ondes dans une vapeur atomique. Chaos et morphogénèse.

Abstract : The interaction of two counterpropagating electromagnetic waves of same frequency and same linear polarisation induces the generation of new waves when the intensity of these pump beams exceeds a threshold value. These new waves are called instabilities and are emitted in directions different from the direction of propagation of the pump beams. In this PhD work, the instabilities generated by two counterpropagating beams in a Rubidium vapor are studied in two different experiments. In the first experiment, an intense beam propagates in the non linear medium and is reflected on a feedback mirror so that the reflected field propagates also in the non linear medium. When the intensity exceeds a threshold value and the frequency of the beam is tuned near the D1 transition of Rubidium 85, off-axis instabilities are emitted. They propagate between the non linear medium and the feedback mirror and are reflected by this mirror. Their transverse pattern in far field consists of concentric rings, double spots or of 'Flowers' consisting of an odd number of spots ('petals') distributed on a circle. The theoretical and experimental study of such instabilities makes it possible to predict and observe the number of petals versus the different parameters of the system In particular, these studies show also that the intensity of the instabilities could be either static, periodic or chaotic. In the second experiment, the laser beam is divided in two pump beams conterpropagating in the non linear medium. No feedback mirror is used. The far field patterns consist of rings, double spots or hexagons. This experiment shows important analogies with hydrodynamics.
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  • HAL Id : tel-00011896, version 1

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Agnès Maître. Instabilités crées par mélange à quatre ondes dans une vapeur atomique. Chaos et morphogénèse.. Physique Atomique [physics.atom-ph]. Ecole Polytechnique X, 1994. Français. ⟨tel-00011896⟩

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