ATOMIC COHERENCE EFFECTS IN 4-LEVEL SYSTEMS - DOPPLER-FREE ABSORPTIONWITHIN AN ELECTROMAGNETICALLY-INDUCED-TRANSPARENCY WINDOW

We report here an effect in a four-level ladderlike system, which is i n contrast to the usual quantum interference effects such as electroma gnetically induced transperency (EIT) or coherent population trapping: we predict the occurrence of a narrow absorption peak within the EIT window when an EIT atomic system interacts with an additional driving rf field. The Doppler-free-central absorption appears when the three-p hoton resonance condition is satisfied. In the limit of the rf field s trength Omega(rf)-->0, the usual EIT profile is recovered.