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arXiv:1210.1310 · Europhysics Letters (EPL)

Sub-Doppler laser cooling of fermionic 40 K atoms in three-dimensional gray optical molasses

Diogo Fernandes, F. Sievers, N. Kretzschmar, Saijun Wu, Ch Salomon, Frédéric Chevy

Published Dec 1, 2012• 108 citations
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Abstract

We demonstrate sub-Doppler cooling of 40 K on the D 1 atomic transition. Using a gray-molasses scheme, we efficiently cool a compressed cloud of 6.5 × 10 8 atoms from ∼4 mK to 20 μ K in 8 ms. After transfer to a quadrupole magnetic trap, we measure a phase space density of ∼10 −5 . This technique offers a promising route for fast evaporation of fermionic 40 K.

DOI
10.1209/0295-5075/100/63001

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We demonstrate sub-Doppler cooling of 40 K on the D 1 atomic transition. Using a gray-molasses scheme, we efficiently cool a compressed cloud of 6.5 × 10 8 atoms from ∼4 mK to 20 μ K in 8 ms. After transfer to a quadrupole magnetic trap, we measure a phase space density of ∼10 −5 . This technique offers a promising route for fast evaporation of fermionic 40 K.
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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 ${10}^{11}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.
We report on simultaneous sub-Doppler laser cooling of fermionic 6Li^{6}\mathrm{Li}6Li and 40K^{40}\mathrm{K}40K using the D1{D}_{1}D1​ optical transitions. We compare experimental results to a numerical simulation of the cooling process applying a semiclassical Monte Carlo wave-function method. The simulation takes into account the three-dimensional optical molasses setup and the dipole interaction between atoms and the bichromatic light field driving the D1{D}_{1}D1​ transitions. We discuss the physical mechanisms at play, identify the important role of coherences between the ground-state hyperfine levels, and compare D1{D}_{1}D1​ and D2{D}_{2}D2​ sub-Doppler cooling. In 5 ms, the D1{D}_{1}D1​ molasses phase greatly reduces the temperature for both 6Li^{6}\mathrm{Li}6Li and 40K^{40}\mathrm{K}40K at the same time, with final temperatures of 44 and $11\ensuremath{\mu}\mathrm{K}$, respectively. For both species this leads to a phase-space density close to ${10}^{\ensuremath{-}4}$. These conditions are well suited to direct loading of an optical or magnetic trap for efficient evaporative cooling to quantum degeneracy.
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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 laser cooling of fermionic 40K{}^{40}\mathrm{K}40K atoms, with temperatures down to $(15\ifmmode\pm\else\textpm\fi{}5) \ensuremath{\mu}\mathrm{K},$ for an enriched sample trapped in a magneto-optical trap and additionaly cooled in optical molasses. This temperature is a factor of 10 below the Doppler-cooling limit and corresponds to an rms velocity within a factor of 2 of the lowest realizable rms velocity $(\ensuremath{\sim}{3.5v}_{\mathrm{rec}})$ in three-dimensional optical molasses. Realization of such low atom temperatures, up to now only accessible with evaporative cooling techniques, is an important precursor to producing a degenerate Fermi gas of 40K{}^{40}\mathrm{K}40K atoms.