This paper studies the effects of vacuum-induced coherence （VIC） in a four-level atomic system. The effects of VIC lead to the coherent hole burnings exhibited in the system at some certain points of the Rabi frequency. This is also the reason for the enhancement of the coherent population trapping. In addition, optical bistability occurs in the evolution curves of absorption versus the phase of Rabi frequencies.
In this paper a Y-type four-level atom interacting with external fields system is investigated. It is shown that increasing the control field can reduce the probe absorption, the electromagnetically induced transparency (EIT) and gain can be obtained if the parameters are taken appropriately. An interesting phenomenon is found that the system appears to be probe gain in a large region if the external fields Rabi phases are taken appropriately, too.
