We demonstrate a gravito-optical surface trap for Cs atoms which exploits cooling in an evanescent light wave. About 10 5 atoms were cooled down to 3 mK and formed a sample with a mean height of ϳ20 mm above the surface of a dielectric prism. The trap does not use a magnetic field and leads to very small atomic level perturbations. The excited-state population of the stored atoms is ϳ1.5 3 10 26 and collisional losses are strongly suppressed. [S0031-9007(97)04024-6] PACS numbers: 32.80. Pj, 42.50.Vk The specular reflection of atoms from an evanescent light wave (EW), originally suggested by Cook and Hill in 1982 [1] and first observed in 1987 [2], has attracted great interest to realize mirrors, resonators, and waveguides for atoms [3].Efforts to confine the motion of atoms with the help of EW mirrors have so far focused on the conservative, i.e., nondissipative or coherent case, motivated by the possibility to construct matter-wave resonators [4,5]. Important experimental steps have been made with the observation of cold atoms bouncing in the field of gravity on a flat EW mirror [6] and with the demonstration of the confinement of atoms in a gravito-optical cavity based on a curved EW [7].Recent work has shown that the reflection of atoms from an EW can also act in a dissipative way [8-12]: Inelastic reflection processes can efficiently extract energy from the atomic motion which opens a way to cool atoms in novel gravito-optical traps with the prospect to obtain very dense samples [9,10]. Experimentally, single cooling EW reflection processes were studied with a thermal Na atomic beam at grazing incidence [11] and with a cold ensemble of Cs atoms dropped onto the EW at normal incidence [12].In this Letter, we present a novel gravito-optical surface trap (GOST) in which we use EW cooling to store an ensemble of Cs atoms closely above a flat dielectric surface. In our trap, schematically shown in Fig. 1, horizontal confinement is provided by the conservative optical dipole potential of a hollow, cylindrical laser beam, far blue detuned from the atomic resonance.The EW cooling mechanism in the GOST is based on the splitting of the 2 S 1͞2 ground state of Cs into two hyperfine sublevels with F 3, 4. Because of the much smaller hyperfine splitting of the excited 2 P 3͞2 state, we can model the atom as a three-level scheme [10] with two ground states separated by d hfs ͞2p 9.2 GHz and one excited state. The EW is linearly polarized and tuned to the blue side of both transitions with corresponding frequency detunings d F d ew (for F 3) and d F d ew 1 d hfs (for F 4). The interaction with the EW leads to light shifts of the atomic levels and thus results in repulsive ground-state potentials for the atomic motionwhich depend on the distance z from the surface and the hyperfine state F, but not on the particular magnetic substate [10]. Here G 2p 3 5.3 MHz and l 852 nm denote the natural linewidth and the wavelength of the optical transition, and I 0 and L represent the maximum intensity and the decay length of the E...

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Ultracold cesium atoms are stored in a novel dipole-force trap, which provides long storage times of spin polarization and facilitates easy Stern-Gerlach selection. The trap consists of a far red-detuned standing light wave, oriented vertically in the field of gravity. By comparing the trapping of a single magnetic substate (F =4, mF =0) with the simultaneous storage of all sublevels, we measure the decay of spin polarization that results from photon scattering of trap light. We furthermore observe spin precession in an optically induced "fictitious magnetic field".

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We demonstrate a novel optical dipole-force trap, which uses an intense blue-detuned conical hollow laser beam together with gravity to confine an ensemble of cesium atoms in a dark spatial region. Efficient cooling is provided by a 3D optical molasses in combination with inelastic reflections from the trapping field. The particular advantages of the trap are its experimental simplicity, the high loading efficiency, and a strong suppression of losses caused by ultracold collisions.