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Frontiers in Optics

Beam propagation in symmetric magnetic atomic guides

Karen Rodriguez, Ángela M. Guzmán

Published Jan 1, 2004• 0 citations
Publisher

Abstract

Modal propagation constants for a magnetic atomic waveguide are obtained by WKB quantization. Propagation of cold atomic beams with Gaussian input transverse profile is simulated with the BPM (Beam Propagation Method). The spatial evolution of transverse monomode and multimode profiles is shown. Multimode Gaussian propagation corresponds to reported experimental conditions.

DOI
10.1364/fio.2004.fwh20

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•Jan 1, 2006•0 citations
Modal propagation constants for a magnetic atomic waveguide are obtained by WKB quantization. Propagation of cold atomic beams with Gaussian input transverse profile is simulated with the BPM (Beam Propagation Method). The spatial evolution of transverse monomode and multimode profiles is shown. Multimode Gaussian propagation corresponds to reported experimental conditions.
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We perform modal analysis and ultracold atomic beam propagation in hollow optical fibers. The eigenmodes and modal propagation constants of the atomic waveguide are determined by numerical solution of the Schrodinger equation for the center-of-mass motion. The existence of a threshold de Broglie wavelength for the fundamental mode is predicted. Beam propagation is performed by modal expansion of the wavefunction on a truncated basis of eigenmodes of the atomic waveguide. The evolution of the atomic probability density profile along the waveguide for a non cylindrical atomic beam is shown.
We demonstrate the continuous injection and propagation of a cold atomic beam in a high-gradient (up to 2.7kG∕cm2.7\phantom{\rule{0.3em}{0ex}}\mathrm{kG}∕\mathrm{cm}2.7kG∕cm) magnetic guide of 1.7m1.7\phantom{\rule{0.3em}{0ex}}\mathrm{m}1.7m length. Continuous injection is accomplished using a side-loading scheme that involves a sequence of two modified magneto-optic traps. Methods are developed to measure the atomic-flow temperatures and the flux under steady-state conditions. In the high-gradient portion of the guide, the guided atomic beam has a transverse temperature of $420\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}\ifmmode\pm\else\textpm\fi{}40\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$, a longitudinal temperature of 1mK1\phantom{\rule{0.3em}{0ex}}\mathrm{mK}1mK, and an average velocity of order 1m∕s1\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}1m∕s. Using a radio-frequency current of a fixed frequency $\ensuremath{\nu}$ coupled directly into the guide wires, atoms exceeding a transverse energy of $h\ensuremath{\nu}$ can be continuously and selectively removed from the atomic beam.
We have constructed a tapered magnetic atom guide that is integrated with a magneto-optical trap. We have observed atom guiding over distances of up to 20 cm. This integrated strategy is useful for loading atoms into wire guides. We have shown that tapered sections in atom guides. i.e. sections with a variable steepness of the transverse guiding potential, can reflect atoms propagating in the guide. The reflection arises from the adiabatic conservation of the action of the transverse motion; the latter can be used as a control parameter for the atom reflectivity. A method of producing a slow, dark atomic beam of atoms is demonstrated. Atoms that are extracted from a magneto-optical trap using a push beam are quickly pumped into a dark state, which does not interact with the push laser beam. This method is capable of producing atoms with a velocity of order 1 m/s. The slow atoms can be coupled into atom guides. In related research, we observe tunneling resonances of atoms in a 1D optical lattice through the measurement of the magnetization. These resonances manifest themselves as modulations in the magnetization curves. The resonances are shown to be related to the coherence properties of the atoms, and exist only under conditions where the coherence time exceeds the tunneling period of the atoms.
The continuous injection and propagation of a cold atomic beam in a 1.7 m long high-gradient (up to 2.7 kG/cm) magnetic guide is demonstrated. Continuous injection is accomplished by side-loading a magnetic guide with atoms from a sequence of two modified magneto-optical traps. In the high-gradient portion of the guide, the guided atomic beam has a transverse temperature of 420 muK +/- 40 muK, a longitudinal temperature of 1 mK, and an average velocity of order 1 m/s. Essential for the characterization of this steady-state atomic beam, imaging methods compatible with the high field-gradients associated with this magnetic guide have been investigated and developed. By utilizing a probe laser tuned to an open transition, the fluorescence yield per atom is largely fixed throughout the trap volume, independent of the trapping field. This enables a reliable conversion of fluorescence images into atomic-density profiles. Using a fixed radio-frequency (nurf) current coupled directly onto the guide wires, continuous and selective removal of atoms with a transverse energy exceeding hnurf is demonstrated. This removal scheme is the essence of an rf filter, where the number of modes occupied by the atomic flow is reduced. Through sufficient rf-filtering, a near-single-mode guided atomic beam should be achievable, enabling basic atom-interferometric experiments. The extension of this fixed-frequency removal of atoms for continuous position-resolved evaporative cooling in a long magnetic guide is outlined. Employing an innovative gridless technique for performing Direct Simulation of rarefied gas dynamics via Monte Carlo (DSMC), the evaporative cooling process in a guided atomic beam has been simulated. Both fixed-frequency as well as variable-frequency evaporative cooling layouts have been explored and the results of these simulations are presented. The implementation of this gridless technique is presented and its benefits are briefly outlined. In parallel with the development of a continuous-wave atom laser, tools suited to manipulate cold atomic beams in atom guides have been explored. An atomic guide with a tunable aspect ratio, using rf-dressed state potentials, is presented.