Doppler-Free EIT Measurements of Isotope Shifts and Hyperfine Structure of the Potassium-41 6s State

We report Doppler-free measurements and theoretical modeling of the 4s_1/2-4p_1/2-6s_1/2 ladder using electromagnetically induced transparency (EIT).  Using the acquired spectra we have measured the hyperfine splittings and isotope shift of the 6s state of potassium 41, which has not previously been reported.  In the experiments the probe laser is locked to the to one of the 4s_1/2-4p_1/2 hyperfine transitions, while the coupling laser is scanned over the 4p_1/2-6s_1/2 transition. EIT was measured in a magnetically shielded warm vapor cell. The spectra are complicated by the relatively small hyperfine splitting in the 4p_1/2 state. We have explored the effect of relative polarizations of the probe and coupling lasers on the spectra both experimentally and with a multi-level theoretical model including magnetic sublevels.