Doppler-free spectroscopy of metastable Sr atoms using a hollow cathode lamp.

We report on the demonstration of Doppler-free spectroscopy of metastable Sr atoms using a hollow cathode lamp (HCL). We employed a custom Sr HCL, which is filled with a mixture of 0.5 Torr Ne and 0.5 Torr Xe as a buffer gas to suppress velocity changing collisions and increase the populations in all of the (5s5p)P(J=0,1,2) metastable states. We performed frequency modulation spectroscopy for the (5s5p)P-(5s6s)S, (5s5p)P-(5s6s)S, (5s5p)P-(5s5d)D, and (5s5p)P-(5s5d)D transitions with sufficient signal-to-noise ratios for laser frequency stabilization. We also observed the hyperfine transitions of (5s5p)P-(5s5d)D of Sr87. This method would greatly facilitate laser cooling of Sr.