Saturation of resonant third-harmonic generation in phase-matched systems

Abstract

The saturation of resonant third-harmonic generation in absorbing and phase-matched high-density systems is investigated theoretically as well as experimentally. It is shown that the saturation is determined by the two-photon absorption [imaginary part of ${\chi }_S^{\mathrm{}\left(3\right)\mathrm{}}\left(\omega \right)$]. It gives rise to a depletion of the fundamental wave and leads to a change of the level populations and of the effective refractive index destroying the phase matching. This has to be compared with nonresonant systems which are limited by the Kerr effect, the intensity-dependent changes of the refractive index [real part of ${\chi }_S^{\mathrm{}\left(3\right)\mathrm{}}\left(\omega \right)$]. Optimum conditions for resonant systems are reported.