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arXiv:1012.5433 · Journal of Experimental and Theoretical Physics Letters

Sub-doppler laser cooling of thulium atoms in a magneto-optical trap

D. Sukachev, A. Sokolov, K. Chebakov, A. Akimov, N. Kolachevsky, V. Sorokin

Published Nov 1, 2010• 23 citations
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Abstract

Sub-Doppler laser cooling in a magneto-optical trap for thulium atoms at a wavelength of 410.6 nm has been experimentally studied. Without any dedicated molasses period of sub-Doppler cooling, the cloud of 3 × 106 atoms at a temperature of 25(5) μK was observed. The measured temperature is significantly lower than the Doppler limit of 240 μK for the cooling transition at 410.6 nm. The high efficiency of the sub-Doppler cooling process is due to a near-degeneracy of the Landé g-factors of the lower 4f 136s 2 (J = 7/2) and the upper 4f 125d 3/26s 2 (J = 9/2) cooling levels.

DOI
10.1134/s0021364010220133

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Sub-Doppler laser cooling in a magneto-optical trap for thulium atoms at a wavelength of 410.6 nm has been experimentally studied. Without any dedicated molasses period of sub-Doppler cooling, the cloud of 3 × 106 atoms at a temperature of 25(5) μK was observed. The measured temperature is significantly lower than the Doppler limit of 240 μK for the cooling transition at 410.6 nm. The high efficiency of the sub-Doppler cooling process is due to a near-degeneracy of the Landé g-factors of the lower 4f 136s 2 (J = 7/2) and the upper 4f 125d 3/26s 2 (J = 9/2) cooling levels.
We demonstrated laser cooling and trapping of thulium atoms at sub-Doppler temperatures in a magneto-optical trap (MOT). Up to 3 × 10 6 thulium atoms were trapped in the MOT at temperatures down to 25(5) μK which is approximately 10 times lower than the Doppler limit. The lifetime of atoms in the MOT varied between 0.3–1.5 s and was restricted mostly by optical leaks from the upper cooling level. The lower limit for the leaking rate was estimated to be 22(6) s −1 . Due to a big magnetic moment of Tm atoms, a part of them were trapped in a magnetic trap from the quadrupole field of the MOT. We observed about 3 × 10 4 purely magnetically trapped atoms at temperature of 25 μK with a lifetime in the trap of 0.5 s. Also we set up a “dark” MOT consisting of six crossed hollow beams which increased the number of trapped atoms by a factor of 5 leading to 1.5 × 10 7 atoms at the expense of higher temperature.
Deep laser cooling of atoms, ions, and molecules facilitates the study of fundamental physics as well as applied research. In this work, we report on the narrow-line laser cooling of thulium atoms at the wavelength of 506.2nm with the natural linewidth of 7.8kHz, which widens the limits of atomic cloud parameters control. Temperatures of about 400nK, phase-space density of up to 3.5×10−4 and 2×106 number of trapped atoms were achieved. We have also demonstrated formation of double cloud structure in an optical lattice by adjusting parameters of the 506.2nm magneto-optical trap. These results can be used to improve experiments with BEC, atomic interferometers, and optical clocks.
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 ${10}^{11}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.
The possibility of laser cooling of thulium atoms is considered. The hyperfine structure of almost cyclic 4f 13 6s 2 ( J g = 7/2) ↔ 4f 12 5d 3/2 6s 2 ( J e = 9/2) and 4f 13 6s 2 ( J g = 7/2) ↔ 4f 12 5d 5/2 6s 2 ( J e = 9/2) transitions at 410.6 and 420.4 nm, respectively, is studied by the method of sub-Doppler saturation spectroscopy in counterpropagating laser beams. The hyperfine splitting of excited levels involved in these transitions is measured and the natural linewidths of these transitions are determined. The structure of the neighbouring 4f 13 6s6p ( J e = 5/2) and 4f 12 5d 5/2 6s 2 ( J e = 7/2) levels is studied for the first time by this method. The decay probabilities of the J e = 9/2 levels via channels removing atoms from the cooling cycle are calculated. It is found that the branching ratio for the strong transition at 410.6 nm ( A = 6 × 10 7 s -1 ) is smaller than 2 × 10 -5 , which makes this transition most promising for laser cooling. The laser cooling of atoms in a Zeeman cooler at this transition is simulated. The possibility of using a laser-cooled cloud of thulium atoms to study the metrological transition at 1.14 μm is discussed.
We demonstrate sub-Doppler laser cooling and magneto-optical trapping of the rare earth element Holmium. Atoms are loaded from an atomic beam source and captured in six-beam σ+−σ−σ_+ - σ_-σ+​−σ−​ molasses using a strong J=15/2↔J=17/2J=15/2 \leftrightarrow J=17/2J=15/2↔J=17/2 cycling transition at λ=410.5nmλ=410.5 \rm nmλ=410.5nm. Due to the small difference in hyperfine splittings and Landé ggg-factors in the lower and upper levels of the cooling transition the MOT is self-repumped without additional repump light, and deep sub-Doppler cooling is achieved with the magnetic trap turned on. We measure the leakage out of the cycling transition to metastable states and find a branching ratio ∼10−5\sim 10^{-5}∼10−5 which is adequate for state resolved measurements on hyperfine encoded qubits.
Second-stage laser cooling of thulium atoms at the 530.7 nm transition with a natural linewidth of 350 kHz offers an interesting possibility to study different regimes of a magneto-optical trap (MOT). The intermediate value of the spectral linewidth of the cooling transition allows the observation of three distinct regimes depending on the intensity and detuning of the cooling beams, namely, the ``bowl-shaped'' regime when light pressure force competes with gravity, the ``double structure'' regime with interplay between the Doppler and polarization-gradient (sub-Doppler) cooling, and the ``symmetric'' regime when Doppler cooling dominates over sub-Doppler cooling and gravity. The polarization-gradient cooling manifests itself by a two-temperature momentum distribution of atoms resulting in a double-structure of the spatial MOT profile consisting of a cold central fraction surrounded by a hot halo. We studied the double structure regime at different saturation parameters and compared observations with calculations based on semiclassical and quantum approaches. The quantum treatment adequately reproduces experimental results if the MOT magnetic field is properly taken into account.