We explore the efficacy of optical pumping on the $5s4d\phantom{\rule{0.16em}{0ex}}^{1}D_{2}\text{\ensuremath{-}}5s8p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}$
(448nm) transition in a magneto-optical trap (MOT) of Sr atoms. The number of trapped atoms is enhanced by a factor of
12.0(6) relative to the case without repumping light, which is six times as large as that obtained using the pumping transition $5s4d\phantom{\rule{0.16em}{0ex}}^{1}D_{2}\text{\ensuremath{-}}5s6p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}$
(717nm). This enhancement is limited by decay pathways that bypass the
5s4d1D2 state, namely $5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}\ensuremath{\rightarrow}5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{1}\ensuremath{\rightarrow}5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{0}$ and $5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}\ensuremath{\rightarrow}5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{2}\ensuremath{\rightarrow}5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$, which account for 8% of the total loss of the trapped atoms. We determine the decay rates for the $5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}\ensuremath{\rightarrow}5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{1}$ and $5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}\ensuremath{\rightarrow}5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{2}$ transitions to be $66(6)\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ and $2.4(2)\ifmmode\times\else\texttimes\fi{}{10}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, respectively. Furthermore, we experimentally demonstrate, when the trap beam diameter is small, escape of atoms in the
5s4d1D2 state, which has a relatively long lifetime of $400\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{s}$, becomes a dominant loss mechanism, and that the
448nm pumping light effectively suppresses this escape. Our findings will contribute to improved laser cooling and fluorescence imaging in cold strontium atom platforms, such as quantum computers based on optical tweezer arrays.
We present a faster repumping scheme for strontium magneto-optical traps operating on the broad 5 s 2 1 S 0 − 5 s 5 p 1 P 1 laser cooling transition. Contrary to existing repumping schemes, we directly address lost atoms that spontaneously decayed to the 5 s 4 d 1 D 2 state, sending them back into the laser cooling cycle by optical pumping on the 5 s 4 d 1 D 2 − 5 s 8 p 1 P 1 transition. We thus avoid the ∼ 100 µ s -slow decay path from 5 s 4 d 1 D 2 to the 5 s 5 p 3 P 1 , 2 states that is part of other repumping schemes. Using one low-cost external-cavity diode laser emitting at 448 nm , we show our scheme increases the flux out of a 2D magneto-optical trap by 60 % compared to without repumping. Furthermore, we perform spectroscopy on the 5 s 4 d 1 D 2 − 5 s 8 p 1 P 1 transition and measure its frequency ν Sr 88 = ( 668917515.3 ± 4.0 ± 25 ) MHz . We also measure the frequency shifts between the four stable isotopes of strontium and infer the specific mass and field shift factors, δ ν SMS 88 , 86 =<
We describe repumping and spectroscopy of laser-cooled strontium (Sr) atoms using the (5s5p) 3 P 2 –(5s4d) 3 D 2 transition. Atom number in a magneto-optical trap is enhanced by driving this transition because Sr atoms that have decayed into the (5s5p) 3 P 2 dark state are repumped back into the (5s 2 ) 1 S 0 ground state. Spectroscopy of 84 Sr, 86 Sr, 87 Sr and 88 Sr improves the value of the (5s5p) 3 P 2 –(5s4d) 3 D 2 transition frequency and determines the isotope shifts for the transition accurately enough to guide laser-cooling experiments with less abundant isotopes.
The theoretical description of the external degrees of freedom of atoms trapped inside a magneto-optical trap (MOT) often relies on the decoupling of the evolution of the internal and external degrees of freedom. That is possible thanks to much shorter timescales typically associated with the first ones. The electronic structure of alkaline-earth atoms, on the other hand, presents ultra-narrow transitions and metastable states that makes such an approximation invalid in the general case. In this article, we report on a model based on open Bloch equations for the evolution of the number of atoms in a magneto-optical trap. With this model we investigate the loading of the strontium blue magneto-optical trap under different repumping schemes, either directly from a Zeeman slower, or from an atomic reservoir made of atoms in a metastable state trapped in the magnetic quadrupolar field. The fluorescence observed on the strong 461~nm transition is recorded and quantitatively compared with the results from our simulations. The comparison between experimental results and calculations within our model allowed to identify the existence of the decay paths between the upper level of the repumping transition and the dark strontium metastable states, which could not be explained by electric dipole transition rates calculated in the literature. Moreover, our analysis pinpoints the role of the atomic movement in limiting the efficiency of the atomic repumping of the Sr metastable states.
We study a novel repumping transition for
88Sr atoms trapped in a 'blue' magneto-optical trap. We show that, while the repumping efficiency is about three orders of magnitude smaller than for traditional schemes, it is sufficient for recycling all atoms, provided the repumping laser beams are arranged to form a 'green' magneto- optical trap (MOT) helping to cool and confine the atoms and preventing their loss. Our main findings are: (i) that the green MOT configuration is able to trap 10 times more atoms in the blue MOT than using the green transition merely as a repump, and (ii) that the atom numbers in the two-color MOT can be balanced through experimental control parameters. The interest of this scheme lies in its capability of reaching low temperature and its suitability for continuous atomic beam generation.
Abstract This article presents spectroscopy results of the 5 s 5 p 3 P 0 → 5 s 5 d 3 D 1 transition in all isotopes of laser cooled Sr atoms and the utility of this transition for repumping application. By employing the 5 s 5 p 3 P 0 → 5 s 5 d 3 D 1 (483 nm) transition in combination with the excitation of 5 s 5 p 3 P 2 → 5 s 6 s 3 S 1 (707 nm) transition, we observe a significant increase (∼13 fold) in the steady state number of atoms in the magneto-optic trap. This enhancement is attributed to the efficient repumping of Sr atoms that have decayed into the dark 5 s 5 p 3 P 2 state by returning them to the ground state 5 s 2 1 S 0 without any loss into the other states. The absolute transition frequencies were measured with an absolute accuracy of 30 MHz. To support our measurements, we performed Fock-space relativistic coupled-cluster calculations of the relevant parameters in Sr To further increase the accuracy of the calculated properties, corrections from the Breit, quantum electrodynamical and perturbative triples were also included. The calculated branching ratio for the repumping state confirms the significantly increased population in the 3 P 1 state. Thereby, leading to an increase of population of atoms trapped due to the enhanced repumping. Our calculated hyperfine-splitting energies are in excellent agreement with the measured values. Moreover, our calculated isotope shifts in the transition frequencies are in good agreement with our measured values.