Dispersion in the anisotropy of optical third-harmonic generation in silicon

We present what are to our knowledge the first theoretical and experimental results for dispersion in the anisotropy (sigma) of chi?((3)) associated with optical third-harmonic generation in Si. The theory is based on a full empirical tight binding band-structure calculation, and the results agree well with our measurements for 0.72 microm < lambda < 1.9 microm. The calculated low-frequency limit of sigma agrees with measurements from the literature better than earlier calculations do. In addition, the dispersion in the relative phase of chi1212((3))/chi1111((3)) also shows good agreement with experiment.