Parametric dispersion in electromagnetically induced transparency

We analyze both experimentally and theoretically the phase shift of a fixed coupling laser field in a Λ-type system induced by a tunable probe laser field. The measurements are performed with a heterodyne interferometer on a beam of cesium atoms in the $D_2$ line. This system exhibits electromagnetically induced transparency with a rapidly varying refractive index. The parametric dispersion of the coupling field over a range of six decades of coupling laser intensities is investigated and found to be in accord with the predictions from the semiclassical model.