Amplification of maser radiation involves an interaction between the radiation and the response function of the molecules that exhibit the population inversion. If the molecular response is taken to be quantum-mechanical, it is shown that the matter-radiation interaction must involve both amplitude and phase if a correct treatment of saturation of the population inversion is to be determined. The semi-classical treatment is compared with a fully quantum-mechanical approach, in which the radiation field is also quantized. Amplification and saturation of the maser radiation are accompanied by a partial locking of the phase across the lineshape. Significant variation of the phase under strongly saturated conditions occurs only over frequency intervals much larger than the homogeneous linewidth. The fully quantum-mechanical treatment of the radiation may have important consequences for the initiation of maser action.
Amplification of maser radiation involves an interaction between the radiation and the response function of the molecules that exhibit the population inversion. If the molecular response is taken to be quantum-mechanical, it is shown that the matter–radiation interaction must involve both amplitude and phase if a correct treatment of saturation of the population inversion is to be determined. The semi-classical treatment is compared with a fully quantum-mechanical approach, in which the radiation field is also quantized. Amplification and saturation of the maser radiation are accompanied by a partial locking of the phase across the lineshape. Significant variation of the phase under strongly saturated conditions occurs only over frequency intervals much larger than the homogeneous linewidth. The fully quantum-mechanical treatment of the radiation may have important consequences for the initiation of maser action.