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arXiv:1107.2337 · Physical Review A

Sub-Doppler laser cooling of potassium atoms

Manuele Landini, Sanjukta Roy, L. Carcagní, Dimitrios Trypogeorgos, M. Fattori, M. Inguscio, Giovanni Carlo Modugno

Published Oct 28, 2011• 77 citations
arXivHTMLPDF

Abstract

We investigate the sub-Doppler laser cooling of bosonic potassium isotopes, whose small hyperfine splitting has so far prevented cooling below the Doppler temperature. We find instead that the combination of a dark optical molasses scheme that naturally arises in this kind of system and an adiabatic ramping of the laser parameters allows us to reach sub-Doppler temperatures for small laser detunings. We demonstrate temperatures as low as $25\ifmmode\pm\else\textpm\fi{}3$ $\ensuremath{\mu}$K and $47\ifmmode\pm\else\textpm\fi{}5$ $\ensuremath{\mu}$K in high-density samples of the two isotopes ${}^{39}$K and ${}^{41}$K, respectively. Our findings should find application to other atomic systems.

DOI
10.1103/physreva.84.043432

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We investigate the sub-Doppler laser cooling of bosonic potassium isotopes, whose small hyperfine splitting has so far prevented cooling below the Doppler temperature. We find instead that the combination of a dark optical molasses scheme that naturally arises in this kind of system and an adiabatic ramping of the laser parameters allows us to reach sub-Doppler temperatures for small laser detunings. We demonstrate temperatures as low as 25\ifmmode±\else\textpm\fi325\ifmmode\pm\else\textpm\fi{}325\ifmmode±\else\textpm\fi3 \ensuremathμ\ensuremath{\mu}\ensuremathμK and 47\ifmmode±\else\textpm\fi547\ifmmode\pm\else\textpm\fi{}547\ifmmode±\else\textpm\fi5 \ensuremathμ\ensuremath{\mu}\ensuremathμK in high-density samples of the two isotopes 39{}^{39}39K and 41{}^{41}41K, respectively. Our findings should find application to other atomic systems.
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 report on the simultaneous sub-Doppler laser cooling of 6Li^{6}\mathrm{Li}6Li and 7Li^{7}\mathrm{Li}7Li isotopes using gray molasses operating on their respective D1{D}_{1}D1​ atomic transitions. For 7Li^{7}\mathrm{Li}7Li we show that the sub-Doppler cooling can be achieved with two distinct $\mathrm{\ensuremath{\Lambda}}$-type transitions, where the upper level can be either of the two 22P1/2{2}^{2}\phantom{\rule{0.16em}{0ex}}{P}_{1/2}22P1/2​ hyperfine states. We obtain temperatures of $\ensuremath{\sim}85\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$, with atom numbers of $\ensuremath{\sim}{10}^{8}$, and phase-space densities in the range of ${10}^{\ensuremath{-}6}\ensuremath{-}{10}^{\ensuremath{-}5}$ for both isotopes. These conditions provide a good starting point for loading the mixture into an optical dipole trap and performing evaporative cooling to quantum degeneracy. Our work provides a valuable simplification for the preparation of ultracold $^{6}\mathrm{Li}\ensuremath{-}^{7}\mathrm{Li}$ mixtures, which were proven to be a successful system for the study of impurity physics and Bose-Fermi superfluids.
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 ${10}^{11}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.
Sub-Doppler laser cooling of 39^{39}39K via the 4S$\to$5P transition
We demonstrate sub-Doppler laser cooling of ^{39} 39 K using degenerate Raman sideband cooling via the 4S _{1/2} \rightarrow 1 / 2 → 5P _{1/2} 1 / 2 transition at 404.8 nm. By using an optical lattice in combination with a magnetic field and optical pumping beams, we obtain a spin-polarized sample of up to 5.6 \times 10^{7} 5.6 × 10 7 atoms cooled down to a sub-Doppler temperature of 4 \upmu μ K, reaching a peak density of 3.9 \times 10^{9} 3.9 × 10 9 atoms/cm ^{3} 3 , a phase-space density greater than 10^{-5} 10 − 5 , and an average vibrational level of \langle \nu \rangle=0.6 ⟨ ν ⟩ = 0.6 in the lattice. This work opens up the possibility of implementing a single-site imaging scheme in a far-detuned optical lattice utilizing shorter wavelength transitions in alkali atoms, thus allowing improved spatial resolution.
In sub-Doppler laser cooling of atoms, the lowest temperatures are known to occur at low (but not too low) excitation, and the temperature increases roughly linearly with laser intensity. However, under conditions for which the lowest temperatures are obtained, the small velocity capture range and low optical pumping rate limit the number of atoms that can be collected into the cold sample. In this study, we present measurements for laser cooling of Rb and metastable He atoms for two counterpropagating laser beams with orthogonal linear polarization (lin\ensuremath{\perp}lin) over a wide range of saturation parameters and laser detunings, together with results of semiclassical (Fokker-Planck) and quantum density matrix calculations. We find that at higher laser intensity, a larger number of atoms can be collected into a final velocity distribution that is significantly narrower than that given by a linear extrapolation vs the square root of laser intensity. Two cooling mechanisms are at work: the sub-Doppler Sisyphus mechanism and also Doppler cooling. Under certain conditions that we discuss, Doppler cooling aids Sisyphus cooling by collecting high-velocity atoms beyond the Sisyphus capture range. He∗{\mathrm{He}}^{*}He∗ presents a rather unusual situation in that the minimum average kinetic energy (Eav){(E}_{\mathrm{av}})(Eav​) for purely Sisyphus cooling is comparable to the minimum value of Eav{E}_{\mathrm{av}}Eav​ with Doppler cooling alone.