Theory of Second-Harmonic Generation at Microwave Frequencies by Paramagnetic Materials

A quantum-mechanical theory of second-harmonic generation in paramagnetic materials is developed when the incoming radiation is in the microwave range, and it is shown to be in agreement with the experimental results of other authors. The theory is also compared with a density-matrix approach based on phenomenological relaxation times, previously developed by Boscaino et al. A qualitative model of the phenomenon of second-harmonic generation is discussed, and as a by-product of this work, approximate analytical solutions are obtained of the eigenvalue problem of a set of two-level systems coupled to a strong monocromatic field.