Parametric dispersion in coherent population trapping systems

Summary form only given. We report on the investigation of the parametric phase shift of a fixed coupling laser field in a lambda type three level system. The parameter under variation is the frequency of the probing field acting on the second transition. In the vicinity of the two photon resonance in this coherent population trapping system the phase of both fixed coupling field and swept probe field undergo a very rapid change. Outside this relatively narrow structure the phase of the probe field follows the usual one photon dispersion line. A semi-classical density matrix model yields analytically expressions for the parametric dispersion and absorption. The system we used was a three level system in the D2 line of cesium obtained from a cesium atomic beam. The phase shift of the coupling beam was measured with a three beam heterodyne interferometer relative to an off-resonant reference laser field. This setup is less sensitive to acoustic and vibrational noise than a homodyne interferometer and provides a reasonable signal to noise ratio even at coupling powers of a few nW. The power of both coupling and probe field was varied over several orders of magnitude and the obtained phase shifts are in good agreement with the semi-classical model.