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Quantum Electronics and Laser Science Conference

High-density cooling of cesium atoms in the gravito-optical surface trap

I. Manek, U. Moslener, M. Hammes, Rudolf Grimm

Published Jan 20, 2003• 0 citations
Publisher

Abstract

Summary form only given. The gravito-optical surface trap (GOST) confines cesium atoms by a blue-detuned repulsive evanescent wave (EW) in combination with gravity. Horizontal confinement is provided by the conservative optical dipole potential of a hollow, cylindrical laser beam, far blue-detuned from resonance. For preparing a very cold and dense cesium gas in the GOST, we use evanescent wave 'Sisyphus' cooling, which is based on inelastic reflections of the atoms from the EW. Using this method we have obtained a temperature of 3 /spl mu/K, which corresponds to an average distance of the atoms from the surface of 19 /spl mu/m. As a very important feature for attaining very high densities, EW cooling allows one to keep the atoms predominantly in the absolute ground state, thus avoiding trap loss by light assisted collisions.

DOI
10.1109/qels.1999.807336

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Summary form only given. The gravito-optical surface trap (GOST) confines cesium atoms by a blue-detuned repulsive evanescent wave (EW) in combination with gravity. Horizontal confinement is provided by the conservative optical dipole potential of a hollow, cylindrical laser beam, far blue-detuned from resonance. For preparing a very cold and dense cesium gas in the GOST, we use evanescent wave 'Sisyphus' cooling, which is based on inelastic reflections of the atoms from the EW. Using this method we have obtained a temperature of 3 /spl mu/K, which corresponds to an average distance of the atoms from the surface of 19 /spl mu/m. As a very important feature for attaining very high densities, EW cooling allows one to keep the atoms predominantly in the absolute ground state, thus avoiding trap loss by light assisted collisions.
Summary form only given. Optical dipole traps facilitate storage of atoms independent of their magnetic sub-state. Trapping at high densities is thus not limited by two-body collisions changing the spin state of the atom, which in the particular case of cesium have so far prevented the attainment of Bose-Einstein condensation in magnetic traps. For evaporative cooling of Cs we use the gravito-optical surface trap (GOST) which consists of a horizontal evanescent-wave (EW) atom mirror in combination with a blue-detuned hollow beam (HE) for transverse confinement. Starting conditions for evaporative cooling are prepared by optical cooling, based on inelastic reflections from the EW.
We propose a novel gravito-optical surface trap (GOST) for neutral atoms based on one-dimensional intensity gradient cooling. The surface optical trap is composed of a blue-detuned reduced semi-Gaussian laser beam (SGB), a far-blue-detuned dark hollow beam and the gravity field. The SGB is produced by the diffraction of a collimated Gaussian laser beam passing through the straight edge of a semi-infinite opaque plate and then is reduced by an imaging lens. We calculate the intensity distribution of the reduced SGB, and study the dynamic process of the SGB intensity-gradient induced Sisyphus cooling for 87 Rb atoms by using Monte Carlo simulations. Our study shows that the proposed GOST can be used not only to trap cold atoms loaded from a standard magneto-optical trap, but also to cool the trapped atoms to an equilibrium temperature of 3.47 μK from ~120 μK, even to realize an all-optical two-dimensional Bose–Einstein condensation by using optical-potential evaporative cooling.
We discuss our recent experimental demonstrations of three novel atom traps which are all based on the optical dipole force in far-detuned laser light. In the gravito-optical surface trap (GOST), Cs atoms are trapped and cooled by means of an evanescent-wave mirror very closely above a dielectric surface. This trap allows for optical cooling at very high densities and provides good starting conditions for evaporative cooling to quantum degeneracy and the realization of a 2D quantum gas. The conical atom trap (CAT) uses a hollow blue-detuned laser beam in combination with gravity to trap atoms in a dark spatial region. This trap is experimentally simple and provides high loading efficiency, tight confinement, efficient sub-Doppler cooling, and strong suppression of collisional losses. The standing-wave red-detuned (STAR) trap allows one to confine atoms in a shallow optical potential at a very low photon scattering rate, and is thus an interesting tool for studies of the atomic ground-state dynamics in external electro-magnetic fields. We have studied atomic spin polarization in this trap.
We report on cooling of an atomic caesium gas closely above an evanescent-wave atom mirror. At high densities, optical cooling based on inelastic reflections is found to be limited by a density-dependent excess temperature and trap loss due to ultracold collisions involving repulsive molecular states. Nevertheless, very good starting conditions for subsequent evaporative cooling are obtained. Our first evaporation experiments show a temperature reduction from 10 μK down to 300 nK along with a gain in phase-space density of almost two orders of magnitude.
We propose a trap based on an evanescent light wave which is formed on the surface of a conical or a pyramidal cavity in a glass substrate. The far blue-detuned evanescent wave acts as a mirror for atoms, bouncing on it in the gravitational field. In addition, fast and deep Sisyphus cooling in the evanescent wave can be organized for alkali atoms. This leads to the cooling of the atoms down to the recoil-limited temperature and their collection in the field- free region near the bottom of the trap, to form an atomic gas of extremely high density.