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Physical Review A

Doppler-free spectroscopy of metastable calcium in a discharge heat pipe

Dirk P. Hansen, Andreas Hemmerich

Published Aug 2, 2005• 13 citations
Publisher

Abstract

We report on Doppler-free frequency modulation spectroscopy and polarization spectroscopy in a discharge heat pipe of yet unexplored transitions connecting the metastable $^{3}P_{2}(4s4p)$ state of calcium atoms with the multiply excited states $^{3}P_{2}(4p4p)$ and $^{3}D_{3}(4s3d)$. Calcium vapor is efficiently produced in a heat pipe operating in a small Pyrex cell filled with $2\phantom{\rule{0.3em}{0ex}}\text{torr}$ of neon. A dc discharge running at $500\phantom{\rule{0.3em}{0ex}}\mathrm{V}$ efficiently populates the metastable triplet states. Narrow resonances with bandwidths of a few tens of MHz with excellent signal-to-noise ratios are observed, well suited as references for laser frequency stabilization. Examination of the Zeeman shifts lets us determine the Land\'e $g$ factors of the excited states and compare them with the predictions based on the Russel-Saunders coupling scheme. We describe an economic, simple, and robust experimental setup that should work for other alkaline-earth-metal atoms like Mg and Sr, as well as for other solid elements with insufficient vapor pressures at room temperature.

DOI
10.1103/physreva.72.022502

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We report on the demonstration of Doppler-free spectroscopy of metastable Sr atoms using a hollow cathode lamp (HCL). We employed a custom Sr HCL, which is filled with a mixture of 0.5 Torr Ne and 0.5 Torr Xe as a buffer gas to suppress velocity changing collisions and increase the populations in all of the (5s5p)3PJ(J=0,1,2) metastable states. We performed frequency modulation spectroscopy for the (5s5p)3P0−(5s6s)3S1, (5s5p)3P1−(5s6s)3S1, (5s5p)3P2−(5s5d)3D2, and (5s5p)3P2−(5s5d)3D3 transitions with sufficient signal-to-noise ratios for laser frequency stabilization. We also observed the hyperfine transitions of (5s5p)3P2−(5s5d)3D3 of Sr87. This method would greatly facilitate laser cooling of Sr.
We explore an efficient method for preparing large samples of ultracold calcium atoms. An optimized conventional (Doppler-limited) magneto-optical trap collects atoms from a Zeeman cooled atomic beam using a strong dipole transition within the singlet system. This transition is not completely closed thus yielding an intense flux of atoms into the metastable triplet state 3P2.{}^{3}{P}_{2}.3P2​. A second magneto-optical trap sharing the same magnetic-field gradient is superimposed which captures and further cools the metastables using the narrow-band infrared transition ${}^{3}{\stackrel{\ensuremath{\rightarrow}}{{P}_{2}}}^{3}{D}_{3}.$ In our present experiment we were able to prepare $3\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at temperatures below 20 microkelvin within 250 ms. Minor technical improvements of our setup promise to yield above 1010{10}^{10}1010 atoms at submicrokelvin temperatures within 1 s.
Doppler-free two-photon transitions between the 3s3s3s, J=2J=2J=2 metastable state and the subconfiguration $7{d}^{\ensuremath{'}}$ of neon are studied with a pulsed-dye-laser oscillator-amplifier system which was synchronized with a chopped rf discharge in neon vapor. The isotopic shifts for these transitions are measured, and based on these measurements we have made a systematic study of Ne-Ne and Ne-He collisional broadening and shift of these very narrow Doppler-free two-photon resonances.
We demonstrate polarization spectroscopy of an excited state transition in room temperature cesium vapor. An anisotropy induced by a circularly polarized pump beam on the D2 transition is observed using a weak probe on the 6P3/2 to 7S1/2 transition. When the D2 transition is saturated, a sub-natural linewidth feature is observed which theoretical modeling shows is enhanced by Doppler averaging. Polarization spectroscopy provides a simple modulation-free signal suitable for laser frequency stabilization to excited state transitions.
We demonstrate polarization spectroscopy of an excited state transition in room-temperature cesium vapor. An anisotropy induced by a circularly polarized pump beam on the D2 transition is observed using a weak probe on the 6P(3/2)→7S(1/2) transition. At high pump power, a subfeature due to Autler-Townes splitting is observed that theoretical modeling shows is enhanced by Doppler averaging. Polarization spectroscopy provides a simple modulation-free signal suitable for laser frequency stabilization to excited state transitions.
The optical frequency transitions with long-lifetime spin triplet states produced by the two valence electrons in the outermost layer of alkaline earth atoms make calcium atom a promising quantum frequency reference. Electron-shelving detection improves the signal-to-noise ratio of the clock transition spectrum, enabling a calcium atomic beam optical frequency standard. This scheme employs the 1S0-1P1 transition at 423 nm to detect the 1S0-3P1 transition at 657 nm, emphasizing the importance of 423 nm laser stability. While the performance of these clocks is well-studied, research on the atomic beam itself is limited. This study investigates frequency shifts in neutral calcium atoms’ 1S0-1P1 transition at 423 nm and their 1S0-3P1 transition at 657 nm. We measure the frequency shift caused by temperature, laser power, and laser alignment, providing insights to optimize the design of the calcium beam’s physical apparatus.