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. Diffraction-de-bragg, trois faisceaux lasers à 120° ayant des vecteurs de polarisation contenus dans le même plan conduit à la cristallisation des atomes sur un réseau hexagonal Il est naturel de faire de la diffraction de Bragg sur cette structure. La condition de diffraction de Bragg [70] est que la différence entre les vecteurs d'ondes incidents et diffractés appartienne au réseau réciproque du réseau direct (hexagonal) Nous pouvons facilement montrer que les vecteurs de base du réseau réciproque sont des vecteurs différence de deux vecteurs d'ondes k, (i=1,2,3) des faisceaux formant le cristal. En effet, à 2D la partie de fréquence positive du champ total peut s'écrire sous la forme : Figure VII-1 : Directions, 2D) Nous avons montré que la configuration à 2D utilisant

B. Lounis, *. , and P. Verkerk, Université Pierre et Marie Curie Case 74, T 12 -E 01, F-75252 Paris Cedex 05, France (**) Laboratoire de Spectroscopie Hertzienne ( §) , Departement de Physique de l'ENS 24, rue Lhomond, F-75231 Paris Cedex 05, France Abstract Resonant enhancements of phase-conjugate reflectivity are observed in cesium optical molasses. These resonances provide information about the internal and external degrees of freedom of the atoms interacting with the cooling beams, such as dynamical properties and spatial order of atoms localized in the optical potential associated with the light-shifts, or friction coefficient of the cooling force (1D corkscrew molasses). Phase-conjugate reflectivities of the order of 10 -2 and large angular aperture (30°) are observed in 3D optical molasses

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F. Dans-l-'expression-de, orientation (< + >) qui a une contribution négligeable devant le terme de magnétisation <J z >. L'origine de Fp est l'interférence entre l'onde sonde polarisée 03C3 + avec l'onde pompe copropageante qui, en modifiant l'intensité totale I 03C3+ , change la pression de radiation exercée sur un atome dont l'état interne est déterminé par les ondes pompes seules (trace sur 03C3 (0) ). En d'autres termes, F P est associée à la pression de radiation d'une onde polarisée 03C3 + se propageant dans la direction +z et proportionnelle à la somme des taux de départ à partir des sous-niveaux Zeeman |g,m&#x3E, pondérés par les populations 03A0 (0) m de ces sous-niveaux en absence de la sonde : car kv «|03B4'|

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. Le-premier-terme-proportionnel-À-tr, ê* s )03B5~ + s 03C3 (0) ] représente l'absorption linéaire de la sonde. Il est négligeable pour des désaccords assez élevés (|03B4 L > 20393). L'expression de la polarisation non-linéaire est alors, p.218

0. ±. Où, et ~ ± s ) sont les opérateurs sans dimension caractérisant le couplage atomechamps pompes (atome-champ sonde respectivement)

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