Phase control of the probe gain without population inversion in a four-level V model

We propose a new scheme for achieving inversionless gain that depends on both an internal interference effect and external coherent fields. We have found that, owing to the quantum interference effect that originates from spontaneous emissions from two closely lying levels, large inversionless gains can be achieved under appropriate conditions. In particular, in addition to the amplitudes, the phases of the applied fields also affect inversionless gain remarkably. That is to say, we can use the relative phase between two intense fields to control probe gain without inversion. (C) 2002 Optical Society of America.