Optical third-harmonic studies of the dispersion in χ¯(3)for gallium nitride thin films on sapphire

Third-harmonic (TH) transmission spectroscopy was used to study the dispersion in the ${\mathrm{\chi }}_{\mathit{x}\mathit{x}\mathit{x}\mathit{x}}^{\mathrm{}\left(3\right)\mathrm{}}$ element of the third-order susceptibility in wurtzite-phase gallium nitride (GaN) films on sapphire. Analysis of the nonlinear response showed an enhancement in ${\mathrm{\chi }}_{\mathit{x}\mathit{x}\mathit{x}\mathit{x}}^{\mathrm{}\left(3\right)\mathrm{}}$ when the photon energy of the TH field was tuned to the absorption edge, ${\mathit{E}}_{\mathit{g}}$=3.398 eV, with a peak value of 2.7×${10}^{\mathrm{-}11}$ e.s.u. Comparison with previous band-structure calculations of ${\mathrm{\chi }}_{\mathit{x}\mathit{x}\mathit{x}\mathit{x}}^{\mathrm{}\left(3\right)\mathrm{}}$ in wide-band-gap zinc-blende semiconductors shows that the dispersion in GaN is qualitatively similar to the third-order response in ZnSe near the absorption edge.