CENTRAL RESONANCE OF THE MOLLOW ABSORPTION-SPECTRUM - PHYSICAL ORIGIN OF GAIN WITHOUT POPULATION-INVERSION

The physical origin of the central resonance of the Mollow absorption spectrum for a two-level atom is studied. This resonance has a dispersive line shape and is centered in omega'= omega where omega' and omega are the frequencies of the probe and pump beam. We give a diagrammatic interpretation of this resonance and we show that it is associated with two-photon spontaneous emission processes. The amplification of the weak beam can be described by one amplitude which exhibits a resonance in omega = omega'. On the other hand, the absorption is described by two amplitudes, one resonant and one nonresonant. The resonant amplification and absorption amplitudes have effects which are balanced. The interference between the resonant and nonresonant absorption amplitudes breaks this symmetry and permits to achieve amplification. This is an example of pin without population inversion because the transition occurs between equally populated levels both in the bare and dressed atom basis. Examples of lasers working along this scheme have indeed been reported in the literature. We finally show that these ideas can be used to suggest new types of lasers without population inversion.