Resonances in Four-Level N-Scheme Atomic Systems in the Bare and Dressed States Pictures

The theoretical study of three resonances that comprise the probe absorption and dispersion lines in the case of four-level N-scheme atomic systems explains the observed behaviors of these systems under different conditions. In this work we derive the three constituent resonances, study their dynamics as a function of the varying strengths of the interacting fields, and match each resonance in both the bare and dressed states pictures with a microscopic pathway that is followed by the system when absorbing a probe photon. The theoretical understanding of the different resonances and their conditions brings clarity to the absorptive versus non-absorptive and slow and fast versus negative group velocities experienced by a probe passing through a four-level N-scheme medium by changing the strengths of the fields.