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Europhysics Letters (EPL)

A magneto-optical trap for chromium with population repumping via intercombination lines

A.S. Bell, J. Stühler, S. Locher, S. Hensler, J. Mlynek, Tilman Pfau

Published Jan 15, 1999• 45 citations
Publisher

Abstract

We present the realisation of a magneto-optical trap for chromium. 52 Cr atoms are loaded directly from a thermal beam. The trap lifetime is enhanced by using two red lasers to repump population that has decayed, via intercombination lines, to metastable levels back into the cooling cycle. We have measured the wavelengths of these intercombination lines and observed coherent Raman spectra. The observed density of 10 8 cm −3 is currently limited by collisions with the hot beam.

DOI
10.1209/epl/i1999-00140-7

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We present the realisation of a magneto-optical trap for chromium. 52 Cr atoms are loaded directly from a thermal beam. The trap lifetime is enhanced by using two red lasers to repump population that has decayed, via intercombination lines, to metastable levels back into the cooling cycle. We have measured the wavelengths of these intercombination lines and observed coherent Raman spectra. The observed density of 10 8 cm −3 is currently limited by collisions with the hot beam.
We have constructed a magneto-optical trap for chromium atoms. Using trapping light at 425 nm and two repumping lasers tuned to intercombination transitions, over 106{10}^{6}106 atoms were trapped and average densities of over ${10}^{16}{\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$ were obtained. Nonexponential loss of atoms is observed at high densities indicating inelastic collisions between trapped atoms. Over a range of trapping conditions, the density-dependent trap loss rate constant \ensuremath{\beta} is extracted from fluorescence decay curves and found to have values in the range of ${10}^{\ensuremath{-}15}{\mathrm{m}}^{3}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1},$ much larger than is observed in alkali-metal traps.
We have realized a scheme for continuous loading of a magnetic trap (MT). 52Cr{}^{52}\mathrm{Cr}52Cr atoms are continuously captured and cooled in a magneto-optical trap (MOT). Optical pumping to a metastable state decouples atoms from the cooling light. Due to their high magnetic moment $(6 {\ensuremath{\mu}}_{B}),$ low-field seeking metastable atoms are trapped in the magnetic quadrupole field provided by the MOT. Limited by inelastic collisions between atoms in the MOT and in the MT, we load 108{10}^{8}108 metastable atoms at a rate of 108{10}^{8}108 atoms/s below $100 \ensuremath{\mu}\mathrm{K}$ into the MT. Optical repumping after the loading allows us to realize a MT of ground-state chromium atoms.
We report on the production of a cold mixture of fermionic 53Cr^{53}\mathrm{Cr}53Cr and 6Li^{6}\mathrm{Li}6Li atoms delivered by two Zeeman-slowed atomic beams and collected within a magneto-optical trap (MOT). For lithium, we obtain clouds of up to $4\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at temperatures of about $500\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$. A gray optical molasses stage allows us to decrease the gas temperature down to $45(5)\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$. For chromium, we obtain MOTs of more than $1.1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms. The availability of magnetically trappable metastable DDD states, from which PPP-state atoms can radiatively decay onto, enables to accumulate into the MOT quadrupole samples of almost 107{10}^{7}107 53Cr^{53}\mathrm{Cr}53Cr atoms. After repumping DDD-state atoms back into the cooling cycle, a final cooling stage decreases the chromium temperature down to $145(5)\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$. While the presence of a lithium MOT decreases the lifetime of magnetically trapped 53Cr^{53}\mathrm{Cr}53Cr atoms, we obtain, within a 5-s duty cycle, samples of about $4\ifmmode\times\else\texttimes\fi{}{10}^{6}$ chromium and $3\ifmmode\times\else\texttimes\fi{}{10}^{8}$ lithium atoms. Our work provides a crucial step toward the production of degenerate Cr-Li Fermi mixtures.
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 investigate the limiting factors in the standard implementation of the Ca magneto-optical trap. We find that intercombination transitions from the 4s5p1P14s5p{\phantom{\rule{0.16em}{0ex}}}^{1}{P}_{1}4s5p1P1​ state used to repump the electronic population from the 3d4s1D23d4s{\phantom{\rule{0.16em}{0ex}}}^{1}{D}_{2}3d4s1D2​ state severely reduce the trap lifetime. We explore seven alternative repumping schemes theoretically and investigate five of them experimentally. We find that all five of these schemes yield a significant increase in the trap lifetime and consequently improve the number of atoms and peak atom density by as much as $\ensuremath{\sim}20$ times and $\ensuremath{\sim}6$ times, respectively. One of these transitions, at 453 nm, is shown to approach the fundamental limit for a Ca magneto-optical trap with repumping only from the dark 3d4s1D23d4s{\phantom{\rule{0.16em}{0ex}}}^{1}{D}_{2}3d4s1D2​ state, yielding a trap lifetime of $\ensuremath{\sim}5$ s.