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Chinese Physics B

Enhanced optical molasses cooling for Cs atoms with largely detuned cooling lasers*

Di Zhang, Yuqing Li, Yunfei Wang, Yongming Fu, Peng Li, Wenliang Liu, Jizhou Wu, Jie Ma, Liantuan Xiao, Suotang Jia

Published Dec 9, 2019• 4 citations
Publisher

Abstract

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.

DOI
10.1088/1674-1056/ab5fc6

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We present a systematic study of the $\mathrm{\ensuremath{\Lambda}}$-enhanced gray molasses cooling ($\mathrm{\ensuremath{\Lambda}}$-GMC) of cesium atoms on the D2{D}_{2}D2​ transition. Due to the large splitting in the excited hyperfine transitions of the cesium D2{D}_{2}D2​ line, it is relatively simple to implement the $\mathrm{\ensuremath{\Lambda}}$-GMC with a suitable frequency control of the trapping and repumping lasers of the magneto-optical trap (MOT). We achieve a temperature of $1.7\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{K}$ with $3.2\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms. The phase space density of atoms is $1.43\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. Compared to the condition with a bare MOT or a MOT with gray-molasses cooling without $\mathrm{\ensuremath{\Lambda}}$ enhancement, there is an increase in the phase space density by a factor of more than 103{10}^{3}103 and 10, respectively.
Laser cooling to sub-Doppler temperatures by optical molasses is thought to be inhibited in atoms with unresolved, near-degenerate hyperfine structure in the excited state. We demonstrate that such cooling is possible in one to three dimensions, not only near the standard D2{D}_{2}D2​ line for laser cooling, but over a wide range extending to the D1{D}_{1}D1​ line. Via a combination of Sisyphus cooling followed by adiabatic expansion, we reach temperatures as low as 40 $\ensuremath{\mu}$K, which corresponds to atomic velocities a factor of 2.6 above the limit imposed by a single-photon recoil. Our method requires modest laser power at a frequency within reach of standard frequency-locking methods. It is largely insensitive to laser power, polarization and detuning, magnetic fields, and initial hyperfine populations. Our results suggest that optical molasses should be possible with all alkali-metal species.
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We present a simple and general optical cooling method based on 3D degenerate Raman sideband cooling with adiabatic release that goes significantly beyond the density and temperature limitations of optical molasses. In 10 ms we cool a sample of 3×108 cesium atoms to a temperature of 330 nK at a density of 1.1×1011 cm−3, which corresponds to a phase-space density nλdB3=1/500.We further propose to cool atoms or molecules inside an optical cavity that enhances the scattering of blue-detuned photons and show that the dissipative mechanism can be viewed as a cavity-induced generalized Doppler cooling. Since the cooling depends on the atom’s internal level structure only through the photon scattering rate, cavity Doppler cooling is applicable to particles that do not possess a closed optical transition, which may allow one to extend laser cooling to a greater class of atoms or molecules. Large samples are cooled at the same rate as single atoms if the effect of one atom on the cavity resonance frequency is small. We also show how to achieve 3D cooling with a single optical cavity.