Phase Dependence of the Inversionless Gain in a Λ System with Near-Degenerate Levels

Due to interaction with the vacuum of the radiation field, near-degenerate lower levels in a Λ system have an additional coherence term, the spontaneously generated coherence (SGC) term. For such a system, we investigate the effect of the relative phase between two coherent fields on the inversionless gain in the presence of SGC. We find that the inversionless gain, which stems from both spontaneously generated coherence and dynamically induced coherence, can be modulated by the relative phase.