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Europhysics Letters (EPL)

The Temperature of Optical Molasses for Two Different Atomic Angular Momenta

Christoph Gerz, Theodore Hodapp, Poul Jessen, K. M. Jones, William D. Phillips, Christoph Ira Westbrook, Klaus Mølmer

Published Feb 20, 1993• 35 citations
Publisher

Abstract

We have measured the temperature of laser-cooled Rb atoms in optical molasses as a function of laser intensity and detuning. For both 85 Rb and 87 Rb, cooled on the F = 3 → F ' = 4 and F = 2 → F ' = 3 transitions, respectively, the temperatures are proportional to the ratio of laser power and detuning for a wide range of these parameters. We observe a small but significant difference between the two isotopes. We also show the results of three-dimensional semi-classical numerical calculations. Our results favor a model which includes atomic localization in optical standing waves.

DOI
10.1209/0295-5075/21/6/005

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We have measured the temperature of laser-cooled Rb atoms in optical molasses as a function of laser intensity and detuning. For both 85 Rb and 87 Rb, cooled on the F = 3 → F ' = 4 and F = 2 → F ' = 3 transitions, respectively, the temperatures are proportional to the ratio of laser power and detuning for a wide range of these parameters. We observe a small but significant difference between the two isotopes. We also show the results of three-dimensional semi-classical numerical calculations. Our results favor a model which includes atomic localization in optical standing waves.
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.
We have measured the low-density characteristics of a magneto-optical trap for both isotopes of rubidium. Trapped-atom temperatures are measured by the time-of-flight technique and are similar to temperatures in optical molasses. They are below the Doppler limit in most cases and increase linearly with the ratio of laser intensity to laser detuning. A slight difference between the two isotopes is observed. The data agree well with three-dimensional, multistate, semiclassical simulations. Spring constants of the trap are inferred from careful measurements of the size of the trapped-atom cloud. The spring constant is seen to increase with intensity at low intensity, becoming independent of intensity at high intensity. This trend is consistent with sub-Doppler cooling mechanisms.
Temperatures of rubidium atoms confined in a magneto-optical trap have been measured using time-of-flight. By varying the parameters of the trap, we find that lower temperatures are obtained with lower laser intensities, larger laser detunings, lower atomic densities, and with the 85 vs. the 87 isotope. Temperatures are below the Doppler limit (140 {mu}K for Rb) in most cases. By measuring the size of the trapped cloud as well as the temperature, we can infer the spring constant of the trap. For comparison purposes, we can measure temperatures in optical molasses by turning off the magnetic field of the trap.