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Frontiers of Physics

Subrecoil Raman laser cooling of $lt;sup$gt;6$lt;/sup$gt;Li atoms

Liang Zhang, Shichuan Yu, Pengyue Liu, Mengjia Yin, Jian Fan, Haibin Wu

Published Jan 1, 2025• 1 citation
Publisher

Abstract

We realize an efficient one-dimensional Raman cooling of 6Li atoms using 2S→2P Raman transition pulses and 2S→3P ultraviolet (UV) optical repumping. In this cooling scheme, we directly cool an initial cold atomic sample of 350 μK released from a standard magneto-optical trap down to a velocity spread of 0.8 recoil velocity, corresponding to an effective temperature of 4.5 μK, an increase of almost an order of magnitude in the phase space density with respect to ordinary laser sub-Doppler cooling. This technique may provide an ideal initial sub-recoil atomic sample for the application of the large-momentum-transfer in the recoil-sensitive lithium atom interferometers.

DOI
10.15302/frontphys.2025.062203

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Subrecoil Raman laser cooling of lt;suplt;suplt;supgt;6lt;/suplt;/suplt;/supgt;Li atoms
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We report the implementation of a two-dimensional Raman cooling scheme using sequential excitations along the orthogonal axes. Using square pulses, we have cooled a cloud of ultracold cesium atoms down to an rms velocity spread of 0.39(5) recoil velocities, corresponding to an effective transverse temperature of 30nK30\phantom{\rule{0.3em}{0ex}}\mathrm{nK}30nK (0.15Trec)(0.15{T}_{\mathrm{rec}})(0.15Trec​). This technique can be useful to improve cold-atom atomic clocks and is particularly relevant for clocks in microgravity.
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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.
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We report on a realization of sub-Doppler laser cooling of 39K^{39}\mathrm{K}39K atoms using degenerate three-dimensional Raman sideband cooling. We take advantage of the well-resolved excited hyperfine states on the D1{D}_{1}D1​ optical transition to produce spin-polarized samples with $1.4\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at temperatures of 1.8 $\ensuremath{\mu}\mathrm{K}$. The phase-space densities are $\ensuremath{\ge}{10}^{\ensuremath{-}4}$, which significantly improves the initial conditions for a subsequent evaporative cooling step. The presented cooling technique using the D1{D}_{1}D1​ line can be adapted to other atomic species and is applicable to high-resolution imaging schemes in far off-resonant optical lattices.