Large Enhancement of Probe Amplification with Population Inversion in a Four-Level Atomic System with Vacuum-Induced Coherence

In this paper, we discuss and analyze theoretically probe absorption-amplification response in a four-levelcoherent atomic system with vacuum-induced coherence via changing the sign of the parameter f, with f denotingthe ratio of a pair of dipole moments associated with a doublet of closely upper hyperfine sublevels. We find that theamplitude of the probe amplification for the case f=-1 can be about one order of magnitude larger than that achievablefor the case f=1. In addition, with respect to the case f=-1 the probe amplification can be maintained all the timewith weak incoherent pumping for a wide range of the probe detuning.