Bistable behaviors of weak probe light via coherent and incoherent fields

In this paper, the bistable behaviors of weak probe light in a unidirectional ring cavity with a four-level atomic system are discussed. We find that in the presence of spontaneously generated coherence (SGC), the medium is completely phase dependent and the intensity threshold of optical bistability can be controlled. The effect of an incoherent pumping field on optical bistability in the presence and absence of SGC is also analyzed. Moreover, the group index behavior of weak probe light is investigated. We show that the intensity threshold of optical bistability can be controlled by light propagation switching from slow to fast by different parameters of the atomic medium.