Observation of inversionless gain and field-assisted lasing in a nearly ideal three-level cascade-type atomic system

We experimentally study the production of laser gain (phase-insensitive optical amplification) with and without atomic population inversion in a doubly driven three-level system in atomic barium. The external fields are found to create inversionless gain and to enhance gain available in the presence of inversion. Laser oscillation based on enhanced gain is observed. In analysis of the driven system, we identify an irreversible multiphoton process that is intrinsically supportive of gain in the absence of population inversion.