The role of repumping in the magneto-optical trap (MOT) has often been seen as a necessary evil, requiring the introduction of either an electro-optical modulator or extra laser.1,2 However, repumping can be exploited to improve the performance of the MOT,3 or to study further the behavior of such a trap. We report results of a modified MOT using for repumping the D1 transitions 3S1/2 F = 1 to 3Pl/2 F = 1 and F = 2. Because these wavelengths are well displaced from the usual (D2) repump wavelengths, the direct repump influence on the trapping and cooling laser interaction should be significantly reduced. We also expect further possibilities for cold-atom spectroscopy within the MOT. The possibility of a dark trap with this repumping transition has been suggested.3
We investigate the limiting factors in the standard implementation of the Ca magneto-optical trap. We find that intercombination transitions from the
4s5p1P1 state used to repump the electronic population from the
3d4s1D2 state severely reduce the trap lifetime. We explore seven alternative repumping schemes theoretically and investigate five of them experimentally. We find that all five of these schemes yield a significant increase in the trap lifetime and consequently improve the number of atoms and peak atom density by as much as $\ensuremath{\sim}20$ times and $\ensuremath{\sim}6$ times, respectively. One of these transitions, at 453 nm, is shown to approach the fundamental limit for a Ca magneto-optical trap with repumping only from the dark
3d4s1D2 state, yielding a trap lifetime of $\ensuremath{\sim}5$ s.
We present a rate equation model to account for the trapping mechanism in the sodium type-II magneto-optical trap (MOT). A key feature of the model is that excitation from the ground F=1 state to both excited states
F∣IH=1 and
F∣IH=0---possible in sodium because of the small excited state separation---is confirmed to be crucial in avoiding the optical pumping that otherwise inhibits steady-state trapping. Furthermore, we calculate that the mixed polarization typically present in a three-dimensional trapping arrangement gives substantially better one-dimensional trapping performance than pure circular polarization, and that the active role of repumping in the trapping process is important. The model is also applied to the type-I (F=2 to
F∣IH=3) MOT and comparisons are made with experiment in both cases. The possibility of blue-detuned Sisyphus cooling in the F=1 to
F∣IH=1 and 0 transition scheme is considered.
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.
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.