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Chinese Physics Letters

Preliminary Results on the First Optical Molasses of Cs Atoms in China

Jidong Hou, Yi-min Li, Donghai Yang, Yiqiu Wang

Published Jun 1, 1998• 3 citations
Publisher

Abstract

Based on an improved optical system, which consists of six laser diodes with two injection locking stages and control set-ups of the intensity and frequency of the laser, we have achieved the first optical molasses in China. The time-of-flight(TOF) signal of the cloud of Cs atoms has been obtained, and an analytical formula for measuring the temperature of the atomic cloud from its TOF signal is given. The lowest temperature of the cloud of Cs atoms is measured to be 10 μK.

DOI
10.1088/0256-307x/15/6/009

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Based on an improved optical system, which consists of six laser diodes with two injection locking stages and control set-ups of the intensity and frequency of the laser, we have achieved the first optical molasses in China. The time-of-flight(TOF) signal of the cloud of Cs atoms has been obtained, and an analytical formula for measuring the temperature of the atomic cloud from its TOF signal is given. The lowest temperature of the cloud of Cs atoms is measured to be 10 μK.
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
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 report a detailed study of the enhanced optical molasses cooling of Cs atoms, whose large hyperfine structure allows to use the largely red-detuned cooling lasers. We find that the combination of a large frequency detuning of about −110 MHz for the cooling laser and a suitable control for the powers of the cooling and repumping lasers allows to reach a cold temperature of ∼ 5.5 μK. We obtain 5.1× 107 atoms with the number density around 1× 1012 cm−3. Our result gains a lower temperature than that got in other experiments, in which the cold Cs atoms with the temperature of ∼ 10 μK have been achieved by the optical molasses cooling.
Preparation of the optical molasses is one of the key steps for developing a fountain clock.The optical molasses is loaded from a slow atomic beam,and the fluorescence collection method is applied to determine the number of atoms in the optical molasses,meanwhile the time-of-flight(TOF) method is used to measure the temperature of the optical molasses.The number of cold atoms in the optical molasses for loading time of 200 ms is determined to be(8.14±0.2)×107,and the temperature of cold atoms is determined to be(10.72±0.69) μK.
We present the current state of things with the development of the Russian cesium fountain. Design of the physical parts, the laser system, and the optical system is briefly outlined. The preliminary results of the operation and detection with a magnetic-optical trap (MOT) and molasses regimes are described. We have made detailed temperature measurements on laser-cooled cesium atoms, exploring a wide range of laser intensity and detuning. The lowest measured temperature of about 20 /spl mu/K for MOT and 2.2 /spl mu/K for molasses are reached.