Coherence enhanced transient lasing in XUV regime

We report the effect of a coherent drive on transient lasing in three-level Lambda and Xi configurations (c <-> a <-> b). We show that the presence of a resonant coherent drive on the a <-> c optical transition can yield an order of magnitude enhancement of the output laser energy on a a <-> b extreme ultraviolet (XUV) or X-ray transition than with no coherent drive. We demonstrate the crucial role of coherence. a c for the laser power enhancement. Contrary to the forward direction (with respect to the pump laser), where forward gain can be enhanced for some choice of the drive Rabi frequency Omega(c), coherent drive on the ac transition always suppresses the backward gain.