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Physical Review A

Two-color magneto-optical trap for metastable helium

A. S. Tychkov, Jeroen C. J. Koelemeij, T. Jeltes, W. Hogervorst, W. Vassen

Published May 5, 2004• 14 citations
Publisher

Abstract

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 $1083\phantom{\rule{0.3em}{0ex}}\text{nm}$ magneto-optical trap (MOT) and a compressed $389\phantom{\rule{0.3em}{0ex}}\text{nm}$ MOT, a density increase of more than one order of magnitude is achieved within $5\phantom{\rule{0.3em}{0ex}}\text{ms}$. 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 $1\phantom{\rule{0.3em}{0ex}}\text{mK}$ to $0.4\phantom{\rule{0.3em}{0ex}}\text{mK}$. 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 $1083\phantom{\rule{0.3em}{0ex}}\text{nm}$ laser cooling only.

DOI
10.1103/physreva.69.055401

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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×\else\texttimes\fi1088\ifmmode\times\else\texttimes\fi{}{10}^{8}8\ifmmode×\else\texttimes\fi108 atoms at a central density of 5\ifmmode×\else\texttimes\fi1010cm\ensuremath−35\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{\ensuremath{-}3}5\ifmmode×\else\texttimes\fi1010cm\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×\else\texttimes\fi10\ensuremath−6)(4.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6})(4.1\ifmmode×\else\texttimes\fi10\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 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 107{10}^{7}107 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.
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 an experimental study of peak and phase-space density of a two-stage magneto-optical trap (MOT) of 6Li^{6}\mathrm{Li}6Li atoms, which exploits the narrower $2{S}_{1/2}\ensuremath{\rightarrow}3{P}_{3/2}$ ultraviolet (UV) transition at 323 nm following trapping and cooling on the more common D2D2D2 transition at 671 nm. The UV MOT is loaded from a red MOT and is compressed to give a high phase-space density up to $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. Temperatures as low as $33\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ are achieved on the UV transition. We study the density limiting factors and in particular find a value for the light-assisted collisional loss coefficient of $1.3\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm3{\mathrm{cm}}^{3}cm3/s for low repumping intensity.
We realize a magneto-optical trap (MOT) of titanium (Ti) atoms, performing laser cooling on the 498 nm transition between the long-lived 3 d 3 ( F 4 ) 4 s a 5 F 5 metastable state and the 3 d 3 ( F 4 ) 4 p y 5 G 6 o excited state. Without the addition of any repumping light, we observe MOTs of the three stable, I = 0 bosonic isotopes, Ti 46 , Ti 48 , and Ti 50 . Up to 8.30 ( 26 ) × 10 5 Ti 48 atoms are trapped at a maximum density of 1.3 ( 4 ) × 10 11 cm − 3 and at a temperature of 90 ( 15 ) µ K . By measuring the decay of the MOT, we constrain the leakage branching ratio of the cooling transition ( ≤ 2.5 × 10 − 6 ) and the two-body loss coefficient ( ≤ 2 × 10 − 10 cm 3 s − 1 ). Our approach to laser cooling Ti can be applied to other transition metals, enabling a significant expansion of the elements that can be laser cooled.
A cloverleaf magnetic trap is loaded from a magneto-optical trap containing 2 × 10 9 helium atoms in the metastable 2 3 S 1 state. With optical molasses and a spin polarization pulse up to 1.5 × 10 9 atoms are magnetically trapped. The vacuum limited trap lifetime is ∼12 s. Compression in 0.5 s yields a density increase to 8 × 10 9 cm −3 . With rf-forced evaporation using an exponentially shaped frequency sweep with a 5 s time constant the phase space density is increased from 1 × 10 −7 to 4 × 10 −3 . The central density increases to 3 × 10 11 cm −3 , while an increase in elastic collision rate from 40 to 220 s −1 indicates runaway evaporation.