Effect of two-photon radiation absorption on second harmonic generation in crystals

A theoretical analysis is made of the effect of two-photon radiation absorption on second harmonic generation in crystals. The cases of simultaneous absorption of fundamental-frequency radiation and second-harmonic quanta and of simultaneous absorption of two second-harmonic quanta are studied. Graphs of the dependences of the conversion efficiency on the two-phonon absorption coefficient are plotted for these cases. Second harmonic generation in lithium metaniobate crystals is analyzed as an example.