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Physical Review Letters

Two-Color Excitation of Metastable Helium Atoms for Efficient Laser Cooling

Wilbert Rooijakkers, W. Hogervorst, W. Vassen

Published Apr 24, 1995• 25 citations
Publisher

Abstract

Radiation pressure on metastable helium atoms can be increased by an order of magnitude using the combined action of two lasers exciting the $1s2s^{3}S_{1}$- $1s2p^{3}P_{2}$- $1s3d^{3}D_{3}$ cascade. This increase has been measured as a function of laser intensities and is explained in a model based on the optical Bloch equations, the coherences playing a crucial role. Possible applications for deceleration are discussed.

DOI
10.1103/physrevlett.74.3348

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Radiation pressure on metastable helium atoms can be increased by an order of magnitude using the combined action of two lasers exciting the 1s2s3S11s2s^{3}S_{1}1s2s3S1​- 1s2p3P21s2p^{3}P_{2}1s2p3P2​- 1s3d3D31s3d^{3}D_{3}1s3d3D3​ cascade. This increase has been measured as a function of laser intensities and is explained in a model based on the optical Bloch equations, the coherences playing a crucial role. Possible applications for deceleration are discussed.
The optical pumping of atoms is a central process in many applications of atomic physics, such as laser cooling and the precise determination of physical constants. Popular methods for producing metastable helium atoms in supersonic expansions are discharge and electron-impact excitation. However, both approaches yield helium atoms in two metastable states, 21S0{2}^{1}{S}_{0}21S0​ and 23S1{2}^{3}{S}_{1}23S1​, which limits the applications of these highly energetic species. This study presents an experimental scheme in which the 21S0{2}^{1}{S}_{0}21S0​ state population is depleted via excitation to the 41P1{4}^{1}{P}_{1}41P1​ state, using diode-laser radiation. The results suggest nearly 100% depletion efficiency at all beam velocities studied.
A model is presented to calculate optical forces, friction, and diffusion for three-level atoms in a ladder configuration. This model is then applied to the ${2}^{3}{S}_{1}{\ensuremath{-}2}^{3}{P}_{2}{\ensuremath{-}3}^{3}{D}_{3}$ cascade in metastable helium. It is demonstrated that metastable helium atoms can be decelerated within a much shorter distance using cascade excitation by overlapping traveling waves. The equilibrium temperature representing the final width of the velocity distribution is calculated as a function of laser detuning and intensity. This equilibrium temperature is also calculated for the situation of overlapping standing waves (one-dimension), and is found to be well above the Doppler limit for two-level ${2}^{3}{S}_{1}{\ensuremath{-}2}^{3}{P}_{2}$ excitation. Numerical results are compared with existing experimental data.
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 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.
The laser cooling of a helium atomic beam in the 2s3S1 metastable state has been demonstrated. Using a single-mode LNA (La1-x Nd x MgAl11O19) laser resonant with the 2s3S1-2p3P2 transition, the metastable atoms are cooled by the Doppler cooling method combined with the Zeeman-tuning technique. To monitor the velocity distribution, the Doppler profile of the 2p3P2-3d3D3 transition is observed. As a result, the most probable velocity of the metastable helium atoms, which is initially 750 m/s, is decelerated to 90 m/s.
A bright beam of metastable helium atoms is useful in various studies. Laser spectroscopy of helium in an atomic beam can provide a determination of the fine-structure constant and the nuclear charge radius, towards a test of quantum electrodynamics (QED). In these measurements, it is necessary to control not only the internal quantum state but also the translation motion of the helium atom. Here we present a setup producing an intense and speed-adjustable continuous beam of helium atoms. By using a combination of laser cooling, focusing, and deflection, helium atoms are prepared at the single quantum state of 23S1{2}^{\phantom{\rule{0.16em}{0ex}}3}{S}_{1}23S1​ (m=0m=0m=0), with a very narrow distribution of the longitudinal speed ($\ensuremath{\delta}{v}_{z}<\ifmmode\pm\else\textpm\fi{}4.5$ m/s). At the same time, we obtain a flux of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{13}$ (atoms/s)/sr with a fractional fluctuation below 0.02%. The atomic-beam quality was investigated by laser spectroscopy, indicating a considerable improvement over those used in previous helium precision-spectroscopy measurements.