Three-stage laser cooling of Sr atoms using the 5s5pP23 metastable state below Doppler temperatures

We report three-stage laser cooling of Sr atoms using the $5{s}^{2}\phantom{\rule{0.16em}{0ex}}^{1}S_{0}$ ground state and the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$ metastable state. $^{87}\mathrm{Sr}$ atoms are precooled in a magneto-optical trap simultaneously operated on the $5{s}^{2}\phantom{\rule{0.16em}{0ex}}^{1}S_{0}$--$5s5p\phantom{\rule{0.16em}{0ex}}^{1}P_{1}$ transition at 461 nm and on the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$--$5s5d\phantom{\rule{0.16em}{0ex}}^{3}D_{3}$ transition at 496 nm. Atoms optically pumped to the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}(F=13/2)$ state are magneto-optically trapped and cooled down to 2.5(2) $\ensuremath{\mu}\mathrm{K}$ on the $5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{2}$--$5s4d\phantom{\rule{0.16em}{0ex}}^{3}D_{3}$ transition at $2.9\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Using this transition, we investigate one dimensional polarization gradient cooling of $^{88}\mathrm{Sr}$ atoms, which have simple energy levels without hyperfine structure, down to 0.22(18) $\ensuremath{\mu}\mathrm{K}$ or 17 times the recoil temperature. We discuss prospects for continuous generation of ultracold Sr atoms combined with a state transfer technique.