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arXiv:1512.07053 · Physical Review A

Sub-Doppler cooling of sodium atoms in gray molasses

Giacomo Colzi, Gianmaria Durastante, Eleonora Fava, Simone Serafini, Giacomo Lamporesi, Gabriele Ferrari

Published Feb 18, 2016• 45 citations
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Abstract

We report on the realization of sub-Doppler laser cooling of sodium atoms in gray molasses using the ${D}_{1}$ 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 ${10}^{11}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.

DOI
10.1103/physreva.93.023421

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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 (3s2S1/2\ensuremath→3p2P1/2)(3s\phantom{\rule{0.16em}{0ex}}^{2}S_{1/2}\ensuremath{\rightarrow}3p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2})(3s2S1/2​\ensuremath→3p2P1/2​) at 589.8 nm. The technique is applied to samples containing 3\ifmmode×\else\texttimes\fi1093\ifmmode\times\else\texttimes\fi{}{10}^{9}3\ifmmode×\else\texttimes\fi109 atoms, previously cooled to 350\ensuremathμK350\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}350\ensuremathμK in a magneto-optical trap, and it leads to temperatures as low as 9\ensuremathμK9\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}9\ensuremathμK and phase-space densities in the range of 10\ensuremath−4{10}^{\ensuremath{-}4}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 1011cm\ensuremath−3{10}^{11}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}1011cm\ensuremath−3.
We report on the efficient gray molasses cooling of sodium atoms using the D 2 optical transition at 589.1 nm. Thanks to the hyperfine split about 6 Γ between | F ′ = 2 ⟩ and | F ′ = 3 ⟩ in the excited state 3 2 P 3 / 2 , this atomic transition is effective for the gray molasses cooling mechanism. Using this cooling technique, the atomic sample in F = 2 ground manifold is cooled from 700 μ K to 56 μ K in 3.5 ms. We observe that the loading efficiency into magnetic trap is increased due to the lower temperature and high phase space density of atomic cloud after gray molasses. This technique offers a promising route for the fast cooling of the sodium atoms in the F = 2 state .
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 present an efficient scheme to implement a gray optical molasses for sub-Doppler cooling of 6Li^{6}\mathrm{Li}6Li atoms with minimum experimental overhead. To integrate the D1{D}_{1}D1​ light for the gray molasses cooling into the same optical setup that is used for the D2{D}_{2}D2​ light for a standard magneto-optical trap (MOT), we rapidly switch the injection seeding of a slave laser between the D2{D}_{2}D2​ and the D1{D}_{1}D1​ light sources. Switching times as short as 30 $\ensuremath{\mu}\mathrm{s}$ can be achieved, inferred from monitor optical beat signals. The resulting low-intensity molasses cools a sample of $N=9\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms to about 60 $\ensuremath{\mu}\mathrm{K}$. A maximum phase-space density of $\ensuremath{\rho}=1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ is observed. On the same setup, the performance of the GM is compared to that of narrow-line cooling in an ultraviolet (UV) MOT, following the procedure in Sebastian et al. [J. Sebastian, Ch. Gross, K. Li, H. C. J. Gan, W. Li, and K. Dieckmann, Phys. Rev. A 90, 033417 (2014)]. Further, we compare the production of a degenerate Fermi gas using both methods. Loading an optical dipole trap from the gray molasses yields a quantum degenerate sample with $3.3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms, while loading from the denser UV MOT yields $2.4\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms. Where the highest atom numbers are not a priority this implementation of the gray molasses technique yields sufficiently large samples for a comparatively low technical effort.
Following the bichromatic sub-Doppler cooling scheme on the D1{D}_{1}D1​ line of 40{}^{40}40K recently demonstrated in Fernandes et al. [Europhys. Lett. 100, 63001 (2012)], we introduce a similar technique for 7{}^{7}7Li atoms and obtain temperatures of $60\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ while capturing all of the $5\ifmmode\times\else\texttimes\fi{}{10}^{8}$ 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 $\ensuremath{\Lambda}$-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 $\ensuremath{\sim}$${10}^{\ensuremath{-}5}$. This laser cooling offers a promising route for fast evaporation of lithium atoms to quantum degeneracy in optical or magnetic traps.
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.