Pump depletion and saturation of two-photon resonant third-harmonic generation processes

Theoretical efficiency limitations for a two-photon resonant third-harmonic generation (THG) process are analyzed in a treatment which includes all nonlinear processes associated with the two-photon resonance assuming negligible material excitation. Results are presented for both collimated-beam and tight-focusing geometries. The theory is applied to experimental results for THG of CO
2
laser radiation in liquid CO-O
2
mixtures; good agreement is found for the observed saturation in the THG efficiency in a tight-focusing geometry.