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Journal of the Optical Society of America B

Optical molasses

Paul D. Lett, William D. Phillips, S. L. Rolston, Carol E. Tanner, Richard N. Watts, Christoph Ira Westbrook

Published Nov 1, 1989• 412 citations
Publisher

Abstract

We present a summary of the results of a simple two-level theory of Doppler cooling in optical molasses and contrast it with the recent theories of multilevel, polarization-gradient cooling. The effects of single-photon recoil and of trapping in microscopic optical potential wells are also considered. Experiments are described in which the temperature of sodium atoms released from optical molasses is measured and found to be well below the Doppler-cooling limit. Measurements of the temperature dependence on many experimental parameters are found to be in good qualitative agreement with the new theories of polarization-gradient cooling.

DOI
10.1364/josab.6.002084

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As soon as the Doppler cooling limit was broken in 3-D optical molasses,1–3 it was realized that polarization gradient cool- ing4,5 would yield temperatures approaching the one-photon recoil energy.

•May 21, 1990•0 citations

Direct measurement of atomic velocity distribution in optical molasses

Miao Zhu, J. L. Hall

Since the first optical molasses1 came out, the temperature of the atomic vapor in the optical molasses, which was determined by measuring the time of flight (TOF), was found to be well below the Doppler cooling limit,2 which is based on the assumption that a single frequency laser interacts with the two-energy-level atoms. The new theory3 proposes that this lower temperature is due to the nonadiabatic motion of the atoms with Zeeman sublevels in the optical field with polarization gradients. A bimodal speed distribution in optical molasses was also observed.4

•May 21, 1990•0 citations
We present a summary of the results of a simple two-level theory of Doppler cooling in optical molasses and contrast it with the recent theories of multilevel, polarization-gradient cooling. The effects of single-photon recoil and of trapping in microscopic optical potential wells are also considered. Experiments are described in which the temperature of sodium atoms released from optical molasses is measured and found to be well below the Doppler-cooling limit. Measurements of the temperature dependence on many experimental parameters are found to be in good qualitative agreement with the new theories of polarization-gradient cooling.
A theoretical analysis is given for laser cooling of a two-level atom with magnetic sublevels in the presence of polarization gradients. The optical Bloch equations for the multilevel system are solved numerically for four combinations of polarizations in one-dimensional optical molasses. The light-pressure force on the atom as given by a simple two-level theory is recovered in the absence of polarization gradients, whereas a spatial variation of the polarization is found to lead to a strong cooling force for slow atoms. The increased cooling force is responsible for the recent observations of atoms cooled in optical molasses to temperatures an order of magnitude below the Doppler limit.
Optical molasses is a three-dimensional (3-D) configuration of laser beams used to laser-cool and to viscously confine neutral atoms. Atoms laser cooled in optical molasses reach temperatures much lower than the limit given by the original theories of laser cooling based on the Doppler effect. This cooling below the Doppler-cooling limit is now seen as being due to new laser cooling mechanisms not considered in the original theories. The dependence of the atomic temperature on parameters such as laser intensity and detuning shows good agreement between calculations performed in 1-D and experiments performed in 3-D. For cooling of Na and Cs atoms, the lowest observed temperatures correspond to rms velocities between three and four times the single photon recoil velocity. For Cs the temperature is 2.5 ± 0.6 μK and is the lowest temperature ever measured for 3-D cooling.
Since the first optical molasses1 came out, the temperature of the atomic vapor in the optical molasses, which was determined by measuring the time of flight (TOF), was found to be well below the Doppler cooling limit,2 which is based on the assumption that a single frequency laser interacts with the two-energy-level atoms. The new theory3 proposes that this lower temperature is due to the nonadiabatic motion of the atoms with Zeeman sublevels in the optical field with polarization gradients. A bimodal speed distribution in optical molasses was also observed.4