Stability and Group Index Switching in a Four-Level Atomic System

A four-level atomic system is proposed for stability and group index switching in the presence of spontaneously generated coherence. It is found that the transition from optical bistability (OB) to optical multistability (OM) and subluminal to superluminal can be obtained simultaneously. It is shown that the relative phase between applied fields can affect the stability and group index behaviors of weak probe light in the medium. Our proposed model provides a new scheme for simultaneous switching from OB to OM and (sub-to-super) luminal light propagation in an atomic system.