Non-phase-matched optical third-harmonic generation in noncentrosymmetric media: Cascaded second-order contributions for the calibration of third-order nonlinearities

Cascaded second-order contributions to third-harmonic generation are analyzed in detail, and are exploited to obtain reliable reference values for third-order nonlinear optical susceptibilities at several wavelengths. We describe the influence of boundary conditions on the measured properties for the case of high nonlinearity crystals such as KNbO₃ and the organic salt 4-N, N-dimethylamino-4^'-N^'-methylstilbazolium toluene-p-sulfonate (DAST). Wavelength dispersion relations for the electronic third-order susceptibilities derived from third-harmonic generation experiments are discussed. Our reference standards allow a comparison of the third-harmonic generation results with measurements of the Kerr susceptibility determined with degenerate four-wave mixing and the z-scan technique and shows that ${{χ}}⁽³⁾({-}{ω},{ω},{-}{ω},{ω})>~{{χ}}⁽³⁾({-}3{ω},{ω},{ω},{ω})$ in the investigated wavelength range. The investigation of structure-property relationships between the perovskite crystals ${KNbO}₃$ and ${KTaO}₃$ indicates similar bond nonlinearities in these two compounds.