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Japanese Journal of Applied Physics

Visible Observation of Metastable Helium Atoms Confined in an Infrared/Visible Double Resonance Trap

M. Kumakura, Norio Morita

Published Mar 1, 1992• 19 citations
Publisher

Abstract

Using a visible probe laser exciting the 2p3P2→3d3D3 transition (λ=588 nm), we have demonstrated a visible observation of metastable He atoms magnetooptically trapped by an infrared laser resonant with the 2s3S1→2p3P2 transition (λ=1083nm). Detecting visible resonance fluorescence, only 104 atoms confined in a trap with a diameter of 1 mm have been sensitively observed even with the naked eye. It has been found that under a given condition the visible laser operates as a trapping laser and considerably extends the capture range of infrared laser detuning.

DOI
10.1143/jjap.31.l276

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Using a visible probe laser exciting the 2p3P2→3d3D3 transition (λ=588 nm), we have demonstrated a visible observation of metastable He atoms magnetooptically trapped by an infrared laser resonant with the 2s3S1→2p3P2 transition (λ=1083nm). Detecting visible resonance fluorescence, only 104 atoms confined in a trap with a diameter of 1 mm have been sensitively observed even with the naked eye. It has been found that under a given condition the visible laser operates as a trapping laser and considerably extends the capture range of infrared laser detuning.
About 10 4 triplet metastable helium atoms have been trapped in a magneto-optical trap, based on the 2 3 S 1 –2 3 P 2 transition at 1.083 μm (volume 0.1 mm 3 , temperature 1 mK). The losses of the trap are analysed by monitoring the ion current due to Penning collisions either with background gases, or between trapped atoms. The rate coefficient for these collisions between trapped metastable helium atoms has been found to be unusually large, and the crucial role of the quasi-resonant laser light has been experimentally demonstrated. The rate coefficient of the dominant process (He*(2 3 S 1 ) + He*(2 3 P 2 ) Penning collisions) has been measured by two different methods, and found to be K sp ≃ 10 -7 cm 3 s -1 within an estimated factor of 4. The corresponding cross-section is 6 · 10 6 Å 2 .
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.
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.
Laser studies of metastable antiprotonic helium atoms, which we recently initiated by observing a sharp increase of the antiproton annihilation rate induced by laser-stimulated resonant transitions, have been extended. With a single laser tuned to the resonance already found at 597.26 nm, we have now established the time dependence of the upper state population. With two lasers ignited at variable time separation, we also studied the feeding of the upper state from higher atomic levels. The initial populations and level lifetimes of excited exotic atoms were determined.
We present an exhaustive overview of optical absorption and laser-induced fluorescence lines of Ba atoms in liquid and solid helium matrices in visible and near-infrared spectral ranges. Due to the increased density of isolated atoms, we have found a large number of spectral lines that were not observed in condensed helium matrices before. We have also measured the lifetimes of metastable states. The lowest 3D1{}^{3}{D}_{1}3D1​ metastable state has lifetime of 2.6 s and can be used as an intermediate state in two-step excitations of high-lying states. Various matrix-induced radiationless population transfer channels have been identified.