Tunable and polarization-controlled high-contrast bright and dark coherent resonances in potassium.

We demonstrate high-contrast electromagnetically induced absorption (EIA) bright resonances on the D line of K39 with characteristics comparable to those of the electromagnetically induced transparency (EIT) dark resonances observed in the same conditions. EIA is produced by the interaction of a weak probe beam with the atomic ground state driven in a degenerate coherent superposition by either a co- or counter-propagating pump beam. We have obtained an order of magnitude increase of the EIA's contrast with respect to previous similar experiments, performed with other alkalis, without compromising its linewidth.