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arXiv:1305.5480 · Physical Review A

Quantum-interference-enhanced deep sub-Doppler cooling of 39 K atoms in gray molasses

Dipankar Nath, Raghavan K. Easwaran, G Rajalakshmi, C. S. Unnikrishnan

Published Nov 8, 2013• 69 citations
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Abstract

We report enhanced sub-Doppler cooling of the bosonic atoms of ${}^{39}$K facilitated by formation of dark states with the cooling and repumping lasers tuned to the Raman resonance in $\ensuremath{\Lambda}$ configuration near the ${D}_{1}$ transition. A temperature of about 12 $\ensuremath{\mu}$K and phase-space density $>$$2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ is achieved in the two-stage ${D}_{2}$-${D}_{1}$ molasses and spans a very large parameter region where quantum interference persists robustly. We also present results on enhanced radiation heating with a subnatural linewidth (0.07$\ensuremath{\Gamma}$) and a signature Fano-like profile of a coherently driven three-level atomic system. The optical Bloch equations relevant for the three-level atom in a bichromatic light field are solved with the method of continued fractions to show that cooling occurs only for a small velocity class of atoms, emphasizing the need for precooling in the ${D}_{2}$ molasses stage.

DOI
10.1103/physreva.88.053407

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Following the bichromatic sub-Doppler cooling scheme on the D1-line of 40K recently demonstrated in (Fernandes et al. 2012), we introduce a similar technique for 7Li atoms and obtain temperatures of 60 uK while capturing all of the 5x10^8 atoms present from the previous stage. We investigate the influence of the detuning between the the two cooling frequencies and observe a threefold decrease of the temperature when the Raman condition is fulfilled. We interpret this effect as arising from extra cooling due to long-lived coherences between hyperfine states. Solving the optical Bloch equations for a simplified, \Lambda-type three-level system we identify the presence of an efficient cooling force near the Raman condition. After transfer into a quadrupole magnetic trap, we measure a phase space density of ~10^-5. This laser cooling offers a promising route for fast evaporation of lithium atoms to quantum degeneracy in optical or magnetic traps.
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Sub-Doppler laser cooling of40K with Raman gray molasses on the D2{D}_{2}D2​ line
Gray molasses is a powerful tool for sub-Doppler laser cooling of atoms to low temperatures. For alkaline atoms, this technique is commonly implemented with cooling lasers which are blue-detuned from either the D 1 or D 2 line. Here we show that efficient gray molasses can be implemented on the D 2 line of 40 K with red -detuned lasers. We obtained temperatures of , which enables direct loading of atoms from a magneto-optical trap into an optical dipole trap. We support our findings by a one-dimensional model and three-dimensional numerical simulations of the optical Bloch equations which qualitatively reproduce the experimentally observed cooling effects.
We report on the realization of sub-Doppler laser cooling of sodium atoms in gray molasses using the D1{D}_{1}D1​ optical transition $(3s\phantom{\rule{0.16em}{0ex}}^{2}S_{1/2}\ensuremath{\rightarrow}3p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2})$ at 589.8 nm. The technique is applied to samples containing $3\ifmmode\times\else\texttimes\fi{}{10}^{9}$ atoms, previously cooled to $350\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in a magneto-optical trap, and it leads to temperatures as low as $9\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and phase-space densities in the range of ${10}^{\ensuremath{-}4}$. The capture efficiency of the gray molasses is larger than 2/3, and we observe no density-dependent heating for densities up to ${10}^{11}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.
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