Harmonic generation under two-photon resonance conditions

A theoretical analysis is made of the generation of optical harmonics in a nonlinear medium which exhibits a two-photon transition in which the pump and harmonic photons participate. Frequency doubling is discussed in detail. It is shown that free carriers formed as a result of nonlinear absorption may alter significantly the conversion of the fundamental wave into its harmonic. A discussion is given of the applicability of the proposed theory to recent experiments on the generation of the second harmonic of CO₂ laser radiation in tellurium.