Heterodyne measurement of parametric dispersion in electromagnetically induced transparency

Using three phase-locked lasers, we have observed in a three-level system the phase shift of the frequency fixed coupling laser in dependence of the probe laser frequency relative to an off-resonant reference laser. In the vicinity of the two-photon resonance, the system exhibits electomagnetically induced transparency and we show that the dispersion of coupling and probe field shows different spectra if the probe field frequency is tuned. The parametric dispersion of the coupling field is measured on a cesium atomic beam with a heterodyne interferometer over several decades of coupling and probe laser power. Even with coupling powers in the nW range, the interferometer gives a good signal-to-noise ratio. The results are in good agreement with a numerical simulation of a semiclassical model. Analytical expressions for the dispersion are given.