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Journal of the Optical Society of America B

σ_+–σ_− Optical molasses in a longitudinal magnetic field

M. Walhout, Jean Dalibard, S. L. Rolston, William D. Phillips

Published Nov 1, 1992• 55 citations
Publisher

Abstract

We investigate the effect of a longitudinal magnetic field on the orientational cooling mechanism that operates both in σ+–σ− optical molasses and in a magneto-optical trap. For an atom with different Landé g factors in its ground and excited states, single-photon scattering and stimulated Raman scattering are resonant at different field-dependent velocities. The sharp polarization-gradient feature in the force-versus-velocity curve may therefore be translated to a velocity at which the Doppler force is dominant. In addition to this, we find that the momentum diffusion coefficient can grow as the field is increased. By numerically solving a semiclassical Fokker–Planck equation, we show that these effects lead to a field-induced inhibition of efficient sub-Doppler cooling for atoms with unequal g factors. Our treatment also enables us to discuss other phenomena that are accessible only if both Doppler and polarization-gradient effects are included in the calculation.

DOI
10.1364/josab.9.001997

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We investigate the effect of a longitudinal magnetic field on the orientational cooling mechanism that operates both in σ+–σ− optical molasses and in a magneto-optical trap. For an atom with different Landé g factors in its ground and excited states, single-photon scattering and stimulated Raman scattering are resonant at different field-dependent velocities. The sharp polarization-gradient feature in the force-versus-velocity curve may therefore be translated to a velocity at which the Doppler force is dominant. In addition to this, we find that the momentum diffusion coefficient can grow as the field is increased. By numerically solving a semiclassical Fokker–Planck equation, we show that these effects lead to a field-induced inhibition of efficient sub-Doppler cooling for atoms with unequal g factors. Our treatment also enables us to discuss other phenomena that are accessible only if both Doppler and polarization-gradient effects are included in the calculation.
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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.