Saturation of resonant third-harmonic generation due to self-defocusing and a redistribution of the population densities

Using a high-power excimer laser, pumped dye laser third-harmonic generation in strontium was investigated around several two-photon resonances for input intensities which are well above the onset of saturation. The dominant role of self-defocusing and of the time-dependent redistribution of the population densities is demonstrated. Strong stimulated emission on several transitions branching off from the excited state of the two-photon resonance and several competing parametric processes have been observed. Good agreement is obtained with a theoretical model which accounts for the redistribution of the population densities and which shows the importance of the ac Stark effect for the central structure of the high-intensity conversion profiles.