Origin of gain in systems without inversion in bare or dressed states

We analyze the origin of gain in systems that do not exhibit population inversion either in terms of the bare states or dressed states. Such systems involve light amplification by coherence. We show that gain arises from coherence (a) among Fano states in Arkhipkin-Heller-Harris-like systems (b) between the two dressed states in the {Λ} system of Imamoglu, Field, and Harris [Phys. Rev. Lett. 66, 1154 (1991)].