Vacuum induced interference effect in probe absorption in a driven Y-type atom

We have analyzed the properties of weak probe absorption in a closed four- level Y- type system driven by a coherent driving field. The system with two nearly degenerate uppermost levels interacting with the same modes of the vacuum radiation field exhibits the interference effect between the spontaneous decay channels. The phenomenon of this quantum interference can significantly change the absorptive response of the weak probe field. On varying the amplitude of the resonant driving field, in the absence of incoherent pumping in the upper transitions the system can display probe transparency as well as enhanced absorption peak under the condition of resonant detuning of the probe field. With the application of incoherent pumping to the system the phenomenon of symmetric light amplification with two distinct gain peaks one each at the positions of Autler - Townes resonances can be achieved in the presence of the decay- interference effect. We have also shown that the unidirectional incoherent pumping mode is necessary for the evolution of probe gain.