Creation of population inversion by Fano interference in three-level cascade-type system

We demonstrate how Fano interference produces a large population inversion in a three-level cascade-type system involving an autoionizing state. We show that when an intense monochromatic field driving the lower transition of the atom by a two-photon resonant interaction creates a pair of dressed states which then are coupled by a second coherent field to ionization continuum and an autoionizing state, the Fano interference prevents the photoionization from the upper dressed state whereas the second state is quickly exhausted, thus making possible the coherent generation of short-wavelength light free of phase-matching condition. We discuss the incoherent processes which can destroy the Fano interference.