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arXiv:physics/0210060 · Physical Review A

Magneto-optical trap for metastable helium at 389 nm

Jeroen C. J. Koelemeij, R. J. W. Stas, W. Hogervorst, W. Vassen

Published May 29, 2003• 14 citations
arXivHTMLPDF

Abstract

We have constructed a magneto-optical trap (MOT) for metastable triplet helium atoms utilizing the $2{}^{3}{S}_{1}\ensuremath{\rightarrow}3{}^{3}{P}_{2}$ line at 389 nm as the trapping and cooling transition. The far-red-detuned MOT (detuning $\ensuremath{\Delta}=\ensuremath{-}41\mathrm{MHz})$ typically contains few times ${10}^{7}$ atoms at a relatively high $(\ensuremath{\sim}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3})$ density, which is a consequence of the large momentum transfer per photon at 389 nm and a small two-body loss rate coefficient $(2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}<\ensuremath{\beta}<1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}).$ The two-body loss rate is more than five times smaller than in a MOT on the commonly used $2{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{2}$ line at 1083 nm. Furthermore, laser cooling at 389 nm results in temperatures somewhat lower than those achieved using 1083 nm. The 389-nm MOT exhibits small losses due to two-photon ionization, which have been investigated as well.

DOI
10.1103/physreva.67.053406

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We have constructed a magneto-optical trap (MOT) for metastable triplet helium atoms utilizing the 23S1\ensuremath→33P22{}^{3}{S}_{1}\ensuremath{\rightarrow}3{}^{3}{P}_{2}23S1​\ensuremath→33P2​ line at 389 nm as the trapping and cooling transition. The far-red-detuned MOT (detuning \ensuremathΔ=\ensuremath−41MHz)\ensuremath{\Delta}=\ensuremath{-}41\mathrm{MHz})\ensuremathΔ=\ensuremath−41MHz) typically contains few times 107{10}^{7}107 atoms at a relatively high (\ensuremath∼109cm\ensuremath−3)(\ensuremath{\sim}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3})(\ensuremath∼109cm\ensuremath−3) density, which is a consequence of the large momentum transfer per photon at 389 nm and a small two-body loss rate coefficient (2\ifmmode×\else\texttimes\fi10\ensuremath−10cm3/s<\ensuremathβ<1.0\ifmmode×\else\texttimes\fi10\ensuremath−9cm3/s).(2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}<\ensuremath{\beta}<1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}).(2\ifmmode×\else\texttimes\fi10\ensuremath−10cm3/s<\ensuremathβ<1.0\ifmmode×\else\texttimes\fi10\ensuremath−9cm3/s). The two-body loss rate is more than five times smaller than in a MOT on the commonly used 23S1\ensuremath→23P22{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{2}23S1​\ensuremath→23P2​ line at 1083 nm. Furthermore, laser cooling at 389 nm results in temperatures somewhat lower than those achieved using 1083 nm. The 389-nm MOT exhibits small losses due to two-photon ionization, which have been investigated as well.
We report loading of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{9}$ metastable triplet helium atoms in a large magneto-optical trap, using far-red-detuned laser beams. We fully characterized this trap by measuring trap losses and absorption of a probe beam. From the highly nonexponential trap decay we derive Penning ionization loss rate coefficients for two detunings: $5.3(9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}$ at $\ensuremath{-}35\mathrm{MHz}$ and $3.7(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}$ at $\ensuremath{-}44\mathrm{MHz}.$ Also, we find that the loss rate is maximum at $\ensuremath{-}5\mathrm{MHz}$ detuning, where the rate is $1.3(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}{\mathrm{cm}}^{3}/\mathrm{s},$ much larger than recent theoretical and experimental values. In the absence of light the $S\ensuremath{-}S$ ionization rate constant is measured to be $1.3(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}.$
We describe a powerful scheme which combines laser cooling on two transitions of metastable helium to obtain a high phase-space density. By running a sequence of a large 1083nm1083\phantom{\rule{0.3em}{0ex}}\text{nm}1083nm magneto-optical trap (MOT) and a compressed 389nm389\phantom{\rule{0.3em}{0ex}}\text{nm}389nm MOT, a density increase of more than one order of magnitude is achieved within 5ms5\phantom{\rule{0.3em}{0ex}}\text{ms}5ms. After compression, $8\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at a central density of $5\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{\ensuremath{-}3}$ remain, while the temperature of the cloud has been reduced from 1mK1\phantom{\rule{0.3em}{0ex}}\text{mK}1mK to 0.4mK0.4\phantom{\rule{0.3em}{0ex}}\text{mK}0.4mK. The resulting phase-space density $(4.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6})$ is more than one order of magnitude higher than what we achieved by 1083nm1083\phantom{\rule{0.3em}{0ex}}\text{nm}1083nm laser cooling only.
We have studied a cloud of cold metastable helium (He*) atoms interacting with near-resonant light at 1083 nm and 389 nm. The 1083 nm light allows for efficient loading of a large magneto-optical trap (MOT) and the 389 nm light is subsequently used to increase the density and reduce the temperature of the He* cloud during a brief compression stage. Cold collisions in the cloud yield ions and fast metastables which are monitored separately using calibrated microchannel plate (MCP) detectors. We thus measure absolute production rates of ions and fast metastables escaping from the MOT during the various stages of the experiment. We observe that 389 nm optical collisions, apart from Penning ionization, produce a relatively large flux of fast metastables, which we relate to the short-range behaviour of the molecular potentials involved. Furthermore, by rapidly switching between 389 nm and 1083 nm the ratio between the respective two-body loss rate constants is determined. Using these values, together with the observed time dependence of the cloud size, the temporal behaviour of the absolute ion production rate during the compression stage is well reproduced.
We report the magnetic trapping of metastable 3P2{}^{3}{P}_{2}3P2​ atomic strontium. Atoms are cooled in a magneto-optical trap (MOT) operating on the dipole-allowed ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ transition at 461 nm. Decay via ${}^{1}{\stackrel{\ensuremath{\rightarrow}}{{P}_{1}}}^{1}{\stackrel{\ensuremath{\rightarrow}}{{D}_{2}}}^{3}{P}_{2}$ continuously loads a magnetic trap formed by the quadrupole magnetic field of the MOT. Over 108{10}^{8}108 atoms at a density of $8\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3}$ and temperature of 1 mK are trapped. The atom temperature is significantly lower than what would be expected from the kinetic and potential energies of atoms as they are transferred from the MOT. This suggests the occurrence of thermalization and evaporative cooling in the magnetic trap.
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We report the laser cooling and trapping of neutral potassium on an open transition. Fermionic 40{}^{40}40K is captured using a magneto-optical trap (MOT) on the closed $4{S}_{1/2}\ensuremath{\rightarrow}4{P}_{3/2}$ transition at 767 nm and then transferred, with high efficiency, to a MOT on the open $4{S}_{1/2}\ensuremath{\rightarrow}5{P}_{3/2}$ transition at 405 nm. Because the 5P3/25{P}_{3/2}5P3/2​ state has a smaller linewidth than the 4P3/24{P}_{3/2}4P3/2​ state, the Doppler limit is reduced from 145 \ensuremath{\mu}K to 24 \ensuremath{\mu}K, and we observe temperatures as low as 63(6) $\ensuremath{\mu}$K. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of $\ensuremath{\beta}=1.4(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm3{}^{3}3/s near saturation intensity, and estimate an upper bound of $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ cm2{}^{2}2 for the ionization cross section of the 5P5P5P state at 405 nm. The combined temperature and density improvement in the 405 nm MOT is a twenty-fold increase in phase-space density over our 767 nm MOT, showing enhanced precooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405 nm MOT of bosonic 41{}^{41}41K.