Multiphoton ionization of Na atoms in third-harmonic generation by a mode-locked Nd: YAG laser

Effects of multiphoton ionization on third-harmonic generation in phase-matched Na and Xe mixtures have been investigated using a mode-locked Nd: YAG laser with a pulse duration of 28 psec and peak-power densities up to 3 × ${10}^{12}$ W/${\mathrm{cm}}^2$. Analytical expressions accounting for refractive-index changes due to the depletion of neutral Na atoms and the plasma formation are presented which are used to estimate the wave-vector mismatch created by the multiphoton ionization of Na. The multiphoton-ionization probability $W$ for Na has been obtained from measurement of the number of ions formed by the focused laser beam: $W=6.8\mathrm{}\times {10}^{-49\mathrm{}}{I}_0^5$ ${\sec }^{-1\mathrm{}}$ at $\lambda =1.064\mathrm{}$ μm, where $I_0$ is the field intensity in W/${\mathrm{cm}}^2$. A time-dependent calculation using the value of $W$ obtained shows that multiphoton ionization of Na atoms brings about advancing and shortening of the third-harmonic pulse at high input intensities. The experimental results for the third-harmonic generation are in good agreement with calculated results which incorporated the effects of ionization. A comparison of the ionization effects with the quadratic Kerr effect is also presented.