We report three-stage laser cooling of Sr atoms using the $5s^2{}^{1}S_0$ ground state and the $5s5p{}^{3}P_2$ metastable state. ${}^{87}\mathrm{Sr}$ atoms are precooled in a magneto-optical trap simultaneously operated on the $5s^2{}^{1}S_0$–$5s5p{}^{1}P_1$ transition at 461 nm and on the $5s5p{}^{3}P_2$–$5s5d{}^{3}D_3$ transition at 496 nm. Atoms optically pumped to the $5s5p{}^{3}P_2(F=13/2)$ state are magneto-optically trapped and cooled down to 2.5(2) $\mu \mathrm{K}$ on the $5s5p{}^{3}P_2$–$5s4d{}^{3}D_3$ transition at $2.9\mu \mathrm{m}$. 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) $\mu \mathrm{K}$ or 17 times the recoil temperature. We discuss prospects for continuous generation of ultracold Sr atoms combined with a state transfer technique.

• Received 12 December 2020
• Accepted 8 February 2021

DOI:https://doi.org/10.1103/PhysRevA.103.023331