Probe Gain without Probe field in a V-type System with an External Field Coupling Two Upper Levels

We report a new coherence and interference phenomenon in a V-type system with an external field coupling two upper levels. It is found that the probe gain can be generated even when no probe field is applied to the system, we attribute this result to the existence of the external field. By comparing with the conventional probe gain, the probe gain without the probe field is enhanced greatly because of the absence of the population inversion term.