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 the $(5s5p){}^3P_J(J=0,\ 1,\ 2)$ metastable states. We performed frequency-modulation spectroscopy for the $(5s5p){}^3P_0-(5s6s){}^3S_1$, $(5s5p){}^3P_1-(5s6s){}^3S_1$, $(5s5p){}^3P_2-(5s5d){}^3D_2$, and $(5s5p){}^3P_2-(5s5d){}^3D_3$ transitions with sufficient signal to noise ratios for laser frequency stabilization. We also observed the hyperfine transitions of $(5s5p){}^3P_2-(5s5d){}^3D_3$ of $^{87}\mathrm{Sr}$ . This method would greatly facilitate laser cooling of Sr.