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arXiv:1511.00443 · Applied Physics B

A simple 2 W continuous-wave laser system for trapping ultracold metastable helium atoms at the 319.8 nm magic wavelength

R. J. Rengelink, Remy Notermans, W. Vassen

Published Apr 27, 2016• 19 citations
arXivHTMLPDF

Abstract

High-precision spectroscopy on the $$2 \ ^3 S \rightarrow 2 \ ^1 S$$ transition is possible in ultracold optically trapped helium, but the accuracy is limited by the ac-Stark shift induced by the optical dipole trap. To overcome this problem, we have built a trapping laser system at the predicted magic wavelength of 319.8 nm. Our system is based on frequency conversion using commercially available components and produces over 2 W of power at this wavelength. With this system, we show trapping of ultracold atoms, both thermal (~0.2 μk) and in a Bose–Einstein condensate, with a trap lifetime of several seconds, mainly limited by off-resonant scattering .

DOI
10.1007/s00340-016-6395-y

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High-precision spectroscopy on the 2 3S→2 1S2 \ ^3 S \rightarrow 2 \ ^1 S2 3S→2 1S transition is possible in ultracold optically trapped helium, but the accuracy is limited by the ac-Stark shift induced by the optical dipole trap. To overcome this problem, we have built a trapping laser system at the predicted magic wavelength of 319.8 nm. Our system is based on frequency conversion using commercially available components and produces over 2 W of power at this wavelength. With this system, we show trapping of ultracold atoms, both thermal (~0.2 μk) and in a Bose–Einstein condensate, with a trap lifetime of several seconds, mainly limited by off-resonant scattering .
Motivated by recent direct measurement of the forbidden $2\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}\ensuremath{\rightarrow}3\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}$ transition in helium [Thomas et al., Phys. Rev. Lett. 125, 013002 (2020)], where the ac Stark shift is one of the main systematic uncertainties, we propose a dichroic two-photon transition measurement for $2\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}\ensuremath{\rightarrow}3\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}$, which could effectively suppress the ac Stark shift by utilizing magic wavelengths: one magic wavelength is used to realize state-insensitive optical trapping; the other magic wavelength is used as one of the two lasers driving the two-photon transition. Carrying out calculations based on the no-pair Dirac-Coulomb-Breit Hamiltonian with mass shift operator included, we report the magic wavelength of 1265.615 9(4) nm for 4He^{4}\mathrm{He}4He [or 1265.683 9(2) nm for 3He]^{3}\mathrm{He}]3He] can be used to design an optical dipole trap; the magic wavelength of 934.234 5(2) nm for 4He^{4}\mathrm{He}4He [or 934.255 4(4) nm for 3He]^{3}\mathrm{He}]3He] can be as one excitation laser in the two-photon process and the ac Stark shift can be reduced to less than 100 kHz, as long as the intensity of the other excitation laser does not exceed $1\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{3.33333pt}{0ex}}{\text{W/cm}}^{2}$. Alternatively, by selecting detuning frequencies relative to the 23P2\phantom{\rule{0.16em}{0ex}}^{3}P23P state in the region of 82--103 THz, as well as adjusting the intensity ratios of the two lasers, the ac Stark shift in the $2\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}\ensuremath{\rightarrow}3\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}$ two-photon transition can be canceled.
The magic wavelength for an optical lattice for hydrogen atoms that cancels the lowest order ac Stark shift of the $1S\text{\ensuremath{-}}2S$ transition is calculated to be 513 nm. The magnitudes of the ac Stark shift $\mathrm{\ensuremath{\Delta}}E=\ensuremath{-}119$ Hz/(kW/cm2)\text{Hz}/(\text{kW}/{\mathrm{cm}}^{2})Hz/(kW/cm2) and the slope $d\mathrm{\ensuremath{\Delta}}E/d\ensuremath{\nu}=\ensuremath{-}2.77$ Hz/(GHzkW/cm2)\text{Hz}/(\text{GHz} \text{kW}/{\mathrm{cm}}^{2})Hz/(GHzkW/cm2) at the magic wavelength suggest that a stable and narrow-line-width trapping laser is necessary to achieve a deep enough optical lattice to confine hydrogen atoms in a way that gives a small enough light shift for the precision spectroscopy of the $1S\text{\ensuremath{-}}2S$ transition.
We study the magic wavelength for two-photon 1S–nS transitions in a hydrogen and deuterium atom, as well as 2S–nS transitions, where the lower level is the metastable 2S state. At the magic wavelength, the dynamic Stark shifts of the ground and the excited state of the transition coincide, so that the transition frequency is independent of the intensity of the trapping laser field. Experimentally feasible magic wavelengths of transitions with small slopes in the atomic polarizabilities are determined; these are the most stable magic wavelengths against variations of the laser frequency. We provide data for the magic wavelengths for the 1S–nS and 2S–nS transitions in hydrogen and deuterium, with n=2,⋯,8. We also analyze the stability of the elimination of the ac Stark shift at the magic wavelength against tiny variations of the trapping laser frequency from the magic value.
Optical dipole traps (ODT) with far-off-resonance laser are important tools in more and more present cold-atom experiments, which allow confinement of laser-cooled atoms with a long storage time. Particularly, the magic wavelength ODT can cancel the position-dependent spatially inhomogeneous light shift of desired atomic transition, which is introduced by the ODT laser beam. Now it plays an important role in the state-insensitive quantum engineering and the atomic optical clock. To verify the magic wavelength or the magic wavelength combination for D2 line transition of cesium (Cs) and rubidium (Rb) atoms, we calculated and analyzed the light shift of the 133Cs 6S1/2 - 6P3/2 transition for a monochromatic ODT, and also the 87Rb 5S1/2 - 5P3/2 transition for a dichromatic ODT with a laser frequency ratio of 2:1. Also a dichromatic magic-wavelength ODT laser system for 87Rb atoms is proposed and experimentally realized by employing the quasi-phase-matched frequency doubling technique with telecom laser and fiber amplifier.
We demonstrate a simple scheme to reach Bose–Einstein condensation (BEC) of metastable triplet helium atoms using a single-beam optical dipole trap with moderate power of less than 3 W. Our scheme is based on RF-induced evaporative cooling in a quadrupole magnetic trap and transfer to a single-beam optical dipole trap that is located below the magnetic trap center. We transfer 1×1061\times 10^61×106 atoms into the optical dipole trap, with an initial temperature of $$14\,\upmu\hbox{K}$$ , and observe efficient forced evaporative cooling both in a hybrid trap, in which the quadrupole magnetic trap operates just below the levitation gradient, and in the pure optical dipole trap, reaching the onset of BEC with 2×1052\times 10^52×105 atoms and a pure BEC of 5×1045\times 10^45×104 atoms. Our work shows that a single-beam hybrid trap can be applied for a light atom, for which evaporative cooling in the quadrupole magnetic trap is strongly limited by Majorana spin-flips, and the very small levitation gradient limits the axial confinement in the hybrid trap.
We demonstrate a simple scheme to reach Bose–Einstein condensation (BEC) of metastable triplet helium atoms using a single-beam optical dipole trap with moderate power of less than 3 W. Our scheme is based on RF-induced evaporative cooling in a quadrupole magnetic trap and transfer to a single-beam optical dipole trap that is located below the magnetic trap center. We transfer 1×1061\times 10^61×106 atoms into the optical dipole trap, with an initial temperature of $$14\,\upmu\hbox{K}$$ , and observe efficient forced evaporative cooling both in a hybrid trap, in which the quadrupole magnetic trap operates just below the levitation gradient, and in the pure optical dipole trap, reaching the onset of BEC with 2×1052\times 10^52×105 atoms and a pure BEC of 5×1045\times 10^45×104 atoms. Our work shows that a single-beam hybrid trap can be applied for a light atom, for which evaporative cooling in the quadrupole magnetic trap is strongly limited by Majorana spin-flips, and the very small levitation gradient limits the axial confinement in the hybrid trap.