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Physical Review A

Kinetic theory of magneto-optical traps with finite-linewidth lasers

Jin Woo Jun, Ho Seong Lee

Published Nov 15, 2000• 0 citations
Publisher

Abstract

We generalize the kinetic theory of the magneto-optical trap (MOT) to investigate the finite-linewidth effect of cooling lasers. Our results confirm that the polarization-gradient cooling mechanism in the low-velocity domain of the MOT originates from a two-photon process and that the Doppler cooling mechanism in the high-velocity domain stems from a one-photon process. We find that, when we increase the linewidth of the cooling lasers, the cooling temperature increases monotonically and the atomic cloud radius initially decreases quadratically and later increases.

DOI
10.1103/physreva.62.063407

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•Jan 1, 2006•0 citations
We generalize the kinetic theory of the magneto-optical trap (MOT) to investigate the finite-linewidth effect of cooling lasers. Our results confirm that the polarization-gradient cooling mechanism in the low-velocity domain of the MOT originates from a two-photon process and that the Doppler cooling mechanism in the high-velocity domain stems from a one-photon process. We find that, when we increase the linewidth of the cooling lasers, the cooling temperature increases monotonically and the atomic cloud radius initially decreases quadratically and later increases.
A single-particle kinetic theory of the magneto-optical trap (MOT) for cold atoms is developed for a multilevel dipole interaction scheme. The theory analyzes the one-dimensional motion of (3+5)(3+5)(3+5)-level atoms in an inhomogeneous magnetic field and in the field of two counterpropagating circularly polarized laser waves. It is shown that in multilevel interaction schemes two-photon and generally even-order multiphoton processes are responsible for the formation of the narrow velocity and spatial structures in the radiation pressure force. The structures are located at zero velocity and near the origin of the trap. It is found that in the case of a (3+5)(3+5)(3+5)-level atom, single- and two-photon processes produce the two-component structure of the MOT potential well. Near the bottom of a conventional spatially broad potential well caused by one-photon processes, there is a much narrower well caused by two-photon processes. The double-structure MOT potential well is shown to be responsible for the two-component spatial distribution of the trapped atomic cloud. It is proposed that tight confinement of atoms near the narrow bottom of the MOT potential well can be used for the creation of a pointlike source of cold atoms for various applications in atom optics.
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Magneto-optical traps (MOTs) of Er and Dy have recently been shown to exhibit populationwide sub-Doppler cooling due to their near degeneracy of excited- and ground-state Land\'e ggg factors. We discuss here an additional, unusual intra-MOT sub-Doppler cooling mechanism that appears when the total Dy MOT cooling laser intensity and magnetic quadrupole gradient increase beyond critical values. Specifically, anisotropically sub-Doppler-cooled cores appear, and their orientation with respect to the quadrupole axis flips at a critical ratio of the MOT laser intensity along the quadrupole axis versus that in the plane of symmetry. This phenomenon can be traced to a loss of the velocity-selective resonance at zero velocity in the cooling force along directions in which the atomic polarization is oriented by the quadrupole field. We present data characterizing this anisotropic laser cooling phenomenon and discuss a qualitative model for its origin based on the extraordinarily large Dy magnetic moment and Dy's near degenerate ggg factors.
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We have measured the spring constant, friction parameter, and temperature of the motion of caesium atoms in a magneto-optical trap. The results are very different from those predicted by models based on the scattering force, which have been used up to now. We show that the behaviour can be understood in terms of the sub-Doppler cooling mechanisms which have been observed in molasses, involving atomic motion through a polarization gradient. These mechanisms can operate in a trap because the atoms congregate in a region of low magnetic field. The lowest temperature measured was (30 ± 15) μK.
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