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Physical Review A

Magneto-optical trapping of chromium atoms

C. C. Bradley, Jabez J. McClelland, W. R. Anderson, R. J. Celotta

Published Apr 14, 2000• 55 citations
Publisher

Abstract

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 ${10}^{6}$ 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.

DOI
10.1103/physreva.61.053407

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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 1016m\ensuremath−3{10}^{16}{\mathrm{m}}^{\mathrm{\ensuremath{-}}3}1016m\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−15m3s\ensuremath−1,{10}^{\ensuremath{-}15}{\mathrm{m}}^{3}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1},10\ensuremath−15m3s\ensuremath−1, much larger than is observed in alkali-metal traps.
We realize a combined trap for bosonic chromium (52Cr) and rubidium (87Rb) atoms. Initial experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the 87Rb magneto-optical trap (MOT) in the absence and presence of magnetically trapped ground state 52Cr atoms we determine the scattering cross-section of σ inel,RbCr = 5.0 ± 4.0 × 10−18 m2 for light-induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross-section of the excited 5P3/1 state at an ionizing wavelength of 426 nm is measured to be σ P = 1.1 ± 0.3 × 10−21 m2.
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 magneto-optical trapping of fermionic 53Cr^{53}\mathrm{Cr}53Cr atoms. A Zeeman-slowed atomic beam provides loading rates up to $3\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. We present systematic characterization of the magneto-optical trap (MOT). We obtain up to $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms in the steady-state MOT. The atoms radiatively decay from the excited PPP state into metastable DDD states, and, due to the large dipolar magnetic moment of chromium atoms in these states, they can remain magnetically trapped in the quadrupole field gradient of the MOT. We study the accumulation of metastable 53Cr^{53}\mathrm{Cr}53Cr atoms in this magnetic trap. We also report on the simultaneous magneto-optical trapping of bosonic 52Cr^{52}\mathrm{Cr}52Cr and fermionic 53Cr^{53}\mathrm{Cr}53Cr atoms. Finally, we characterize the light-assisted collision losses in this Bose-Fermi cold mixture.
A detailed experimental and theoretical investigation of a magneto-optical trap for caesium atoms is presented. Particular emphasis has been placed on achieving high spatial number densities and low temperatures. Optimizing both of these together enables efficient evaporative cooling from a conservative trap, a procedure which has recently led to the first observations of Bose-Einstein condensation in a dilute atomic vapour. The behaviour of a magneto-optical trap is nominally determined by four independent parameters: the detuning and intensity of the light field, the magnetic field gradient and the number of trapped atoms. A model is presented which incorporates previous treatments into a single description of the trap that encompasses a wide range of its behaviour. This model was tested quantitatively by measuring the temperature of the cloud and its spatial distribution as a function of the four parameters. The maximum density was found to be limited both by the reabsorption of photons scattered within the cloud and by a reduction of the confining force at small light shifts. The nonlinear variation with position of the restoring force was found to be significant in limiting the number of atoms confined to a high density. A maximum density in phase space (defined as the number of atoms in a box with sides of dimension one thermal de Broglie wavelength) of (1.5 ± 0.5) x 10-5 was observed, with a spatial density of 1.5 x 1011 atoms per cm3. Cold collision losses from a caesium magneto-optical trap have been studied with the purpose of assessing their influence on spatial densities. In contrast to previous measurements of similar quantities, these measurements did not require the use of an ultra-low (< 10-10 Torr) background vapour pressure. The dependence of the cold collision loss coefficient β on the trapping intensity was measured to permit identification of the different cold collision processes. The largest loss rates observed were those due to hyperfine structure-changing collisions, with a coefficient β = (2±1) x 10-10cm3s-1. A study is presented of a modified magneto-optical trap in which a fraction of the population is shelved into a hyperfine level that does not interact with the trapping light. In this so-called "dark" magneto-optical trap, improved densities of nearly 1012cm-3 have been previously reported for sodium. The application of the technique to caesium is not straightforward due to the larger excited state hyperfine splittings. A simple theory for caesium is presented and its main predictions verified by measurements of density, number and temperature. A density of nearly 1012cm,-3 was indeed obtained but at a temperature substantially higher than in the conventional magneto-optical trap.