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arXiv:0802.0836 · Physical Review A

Sub-Doppler laser cooling and magnetic trapping of erbium

Andrew J. Berglund, S. A. Lee, Jabez J. McClelland

Published Nov 27, 2007• 42 citations
arXivHTMLPDF

Abstract

We investigate cooling mechanisms in magneto-optically and magnetically trapped erbium. We find efficient sub-Doppler cooling in our trap, which can persist even in large magnetic fields due to the near degeneracy of two Land\'e $g$ factors. Furthermore, a continuously loaded magnetic trap is demonstrated where we observe temperatures below $25\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$. These favorable cooling and trapping properties suggest a number of scientific possibilities for rare-earth-metal atomic physics, including narrow linewidth laser cooling and spectroscopy, unique collision studies, and degenerate bosonic and fermionic gases with long-range magnetic dipole coupling.

DOI
10.1103/physreva.76.053418

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We investigate cooling mechanisms in magneto-optically and magnetically trapped erbium. We find efficient sub-Doppler cooling in our trap, which can persist even in large magnetic fields due to the near degeneracy of two Land'e ggg factors. Furthermore, a continuously loaded magnetic trap is demonstrated where we observe temperatures below 25\ensuremathμK25\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}25\ensuremathμK. These favorable cooling and trapping properties suggest a number of scientific possibilities for rare-earth-metal atomic physics, including narrow linewidth laser cooling and spectroscopy, unique collision studies, and degenerate bosonic and fermionic gases with long-range magnetic dipole coupling.
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 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.
Magneto-optical traps (MOTs) of Er and Dy have recently been shown to exhibit populationwide sub-Doppler cooling due to their near degeneracy of excited- and ground-state Land\'e ggg factors. We discuss here an additional, unusual intra-MOT sub-Doppler cooling mechanism that appears when the total Dy MOT cooling laser intensity and magnetic quadrupole gradient increase beyond critical values. Specifically, anisotropically sub-Doppler-cooled cores appear, and their orientation with respect to the quadrupole axis flips at a critical ratio of the MOT laser intensity along the quadrupole axis versus that in the plane of symmetry. This phenomenon can be traced to a loss of the velocity-selective resonance at zero velocity in the cooling force along directions in which the atomic polarization is oriented by the quadrupole field. We present data characterizing this anisotropic laser cooling phenomenon and discuss a qualitative model for its origin based on the extraordinarily large Dy magnetic moment and Dy's near degenerate ggg factors.
We report on a novel mechanism that allows for strong laser cooling of atoms that do not have a closed cycling transition. This mechanism is observed in a magneto-optical trap (MOT) for erbium, an atom with a very complex energy level structure with multiple pathways for optical-pumping losses. We observe surprisingly high trap populations of over 10(6) atoms and densities of over 10(11) atoms cm(-3), despite the many potential loss channels. A model based on recycling of metastable and ground state atoms held in the quadrupole magnetic field of the trap explains the high trap population, and agrees well with time-dependent measurements of MOT fluorescence. The demonstration of trapping of a rare-earth atom such as erbium opens a wide range of new possibilities for practical applications and fundamental studies with cold atoms.
We report on the experimental realization of a robust and efficient magneto-optical trap for erbium atoms, based on a narrow cooling transition at 583 nm. We observe up to $N=2\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at a temperature of about T=15T=15T=15 $\ensuremath{\mu}$K. This simple scheme provides better starting conditions for direct loading of dipole traps as compared to approaches based on the strong cooling transition alone, or on a combination of a strong and a narrow kHz transition. Our results on Er point to a general, simple, and efficient approach to laser cool samples of other lanthanide atoms (Ho, Dy, and Tm) for the production of quantum-degenerate samples.
We observed efficient sub-Doppler cooling of 85Rb^{85}\mathrm{Rb}85Rb atoms under a nonzero magnetic field in a moving optical lattice formed in a bichromatic magneto-optical trap. Trap laser detunings were biased or set differently for the counterpropagating laser beams so that the atoms could be trapped where the magnetic field was nonzero. We investigated the center position and the temperature of the atomic cloud. We found that the sub-Doppler cooling effect, known to decrease as the nonzero magnetic field increases in a magneto-optical trap, would in fact increase under a particular trap-laser detuning difference. We first derived the condition under theoretical considerations and then verified it experimentally by conducting nondestructive measurement of atomic temperature.