Three-stage laser cooling of Sr atoms using the 5s5p 3P2 metastable state below Doppler temperatures

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