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Journal of Optics B Quantum and Semiclassical Optics

One-dimensional bichromatic standing-wave cooling of cesium atoms

Andrea Camposeo, Marco Anderlini, D. Ciampini, J H M ller, David Wilkowski, E. Arimondo, Helmut Ritsch

Published Apr 1, 2003• 3 citations
Publisher

Abstract

We report the results of an experimental study on the interaction of cooled cesium atoms with the optical field of two standing waves having different wavelengths (852 and 894 nm) and opposite circular polarizations. The spatial modulation of the superposition of the two optical potentials and the polarization properties of this configuration are expected to produce cooling of the atoms and a spatial modulation of their density with the periodicity of the beat of the two wavelengths. We performed temperature measurements of the cesium sample and observed the density distribution of the atoms for several configurations of the standing wave by means of time-of-flight absorption imaging and fluorescence imaging techniques. Experimentally we could not observe a pronounced density modulation on the length scale of the superperiod. Reasons for this are revealed by a one-dimensional numerical simulation including the complexity of the full Zeeman structure of the cesium atoms. That simulation reproduces the experimental results for the temperatures and spatial confinement.

DOI
10.1088/1464-4266/5/2/355

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We report the results of an experimental study on the interaction of cooled cesium atoms with the optical field of two standing waves having different wavelengths (852 and 894 nm) and opposite circular polarizations. The spatial modulation of the superposition of the two optical potentials and the polarization properties of this configuration are expected to produce cooling of the atoms and a spatial modulation of their density with the periodicity of the beat of the two wavelengths. We performed temperature measurements of the cesium sample and observed the density distribution of the atoms for several configurations of the standing wave by means of time-of-flight absorption imaging and fluorescence imaging techniques. Experimentally we could not observe a pronounced density modulation on the length scale of the superperiod. Reasons for this are revealed by a one-dimensional numerical simulation including the complexity of the full Zeeman structure of the cesium atoms. That simulation reproduces the experimental results for the temperatures and spatial confinement.
We present the experimental realization of optical superlattices created by exposing laser cooled 85Rb{}^{85}\mathrm{Rb}85Rb atoms to a bichromatic standing-wave light field. The two light frequencies are tuned close to the D1{D}_{1}D1​ and D2{D}_{2}D2​ lines of 85Rb.{}^{85}\mathrm{Rb}.85Rb. The total optical potential seen by the atoms exhibits a modulation at the spatial beat period of the two light fields in addition to the typical periodicity on the scale of an optical wavelength. The bichromatic light field cools the atoms into the superlattice potential, resulting in a density modulation on a macroscopic scale. We have observed the density modulation directly in one- and three-dimensional superlattice configurations using an absorption imaging technique.
We have investigated the localization of atoms in two configurations of laser fields: monochromatic and bichromatic standing waves. A quasi resonant monochromatic laser field creates a spatially periodic potential field with period equal to half of the wavelength. If the atomic kinetic energy is less than the height of the potential wells then atoms could be trapped in the wells. The motion of atoms in a standing wave is governed by the gradient force, friction force, and momentum diffusion. We studied one dimensional localization of sodium atoms in such a standing wave, as well as the influence of friction and diffusion on the motion of these atoms. The effect of accumulation of atoms at the bottom of potential wells (nodes of a standing wave) was observed. This leads to a new scheme of laser cooling of atoms with a minimum temperature below the Doppler limit.
We study one-dimensional (1D) sideband cooling of cesium atoms strongly confined in a far-detuned optical lattice. The Lamb-Dicke regime is achieved in the lattice direction whereas the transverse confinement is much weaker. The employed sideband cooling method, first studied by Vuleti\ifmmode \acute{c}\else \'{c}\fi{} et al., uses Raman transitions between Zeeman levels and produces a spin-polarized sample. We present a detailed study of this cooling method and investigate the role of elastic collisions in the system. We accumulate 83(5)% of the atoms in the vibrational ground state of the strongly confined motion, and elastic collisions cool the transverse motion to a temperature of $2.8\ensuremath{\mu}\mathrm{K}=0.7\ensuremath{\Elzxh}{\ensuremath{\omega}}_{\mathrm{osc}}{/k}_{B},$ where ${\ensuremath{\omega}}_{\mathrm{osc}}$ is the oscillation frequency in the strongly confined direction. The sample then approaches the regime of a quasi-2D cold gas. We analyze the limits of this cooling method and propose a dynamical change of the trapping potential as a mean of cooling the atomic sample to still lower temperatures. Measurements of the rate of thermalization between the weakly and strongly confined degrees of freedom are compatible with the zero-energy scattering resonance observed previously in weak 3D traps. For the explored temperature range the measurements agree with recent calculations of quasi-2D collisions. Transparent analytical models reproduce the expected behavior for ${k}_{B}T\ensuremath{\gg}\ensuremath{\Elzxh}{\ensuremath{\omega}}_{\mathrm{osc}}$ and also for ${k}_{B}T\ensuremath{\ll}\ensuremath{\Elzxh}{\ensuremath{\omega}}_{\mathrm{osc}}$ where the 2D features are prominent.
This chapter provides a brief review of the recent theoretical and experimental results of channeling of atoms in the standing wave. It considers the channeling (one dimensional localization) of atoms in two configurations of the laser fields: a monochromatic and a bichromatic one dimensional laser fields. A quasi resonant monochromatic laser field creates a spatially periodic potential field with a period equal to a half of the wavelength. When atomic beam crosses standing wave and atoms have a low enough kinetic energy, they are guided into the channels where they move along the channel and oscillate in the transverse direction. To detect this channeling the atoms have been chosen themselves as probes of their position. A scheme of a cooling of atoms through their channeling is based on the adiabatic lowering the standing wave intensity.
We propose a scheme for three-dimensional trapping and cooling of atoms in a bichromatic standing wave formed by two different longitudinal modes of an optical cavity. The atoms are cooled through an efficient Sisyphus mechanism and trapped at those anti-nodes (maxima) of the stronger field where the second weaker field has a node, this leading to a large effective atom-field coupling. Contrary to the case of a single strong standing wave, the final width of the momentum distribution is considerably less than the depth of the optical potential implying long storage times. The scheme offers the possibility of capturing atoms out of the background and achieving high local densities modulated with the beat frequency of the two involved modes. The results from a three-dimensional semi-classical calculation are confirmed by a full QMCWF simulation as well as by a quantum rate equation model in the one-dimensional case in the proper limits.
We show that Sisyphus cooling in a one-dimensional gray optical lattice can be improved by using a bichromatic standing wave that increases the efficiency of cooling as compared to typical lattices. Using the time of flight technique to determine the velocity distribution of all the atoms, we calculate the efficiency by measuring the fraction of all atoms that are cooled by the lattice. We show that the efficiency depends on the relative displacement between the two standing waves formed by the two colors. The optimal configuration of the bichromatic light field yields a 40% improvement in efficiency compared to typical optical lattices, while maintaining the same atomic temperature.