Ultrafast second-harmonic generation spectroscopy of GaN thin films on sapphire

We have performed ultrafast second-harmonic generation spectroscopy of GaN/Al2O3. A formalism was developed to calculate the nonlinear response of thin nonlinear films excited by an ultrashort laser source (Ti:Al2O3), and then used to extract chi(zxx)((2))(omega=2 omega(o)) and chi(xzx)((2))(omega=2 omega(o)) from our SHG measurements over a two-photon energy range of 2.6-3.4 eV. The spectra are compared to theory [J. L. P. Hughes, Y. Wang, and J. E. Sipe, Phys. Rev. B 55, 13 630 (1997)]. A weak sub-band-gap enhancement of chi(zxx)((2))(omega=2 omega(o)) was observed at a two-photon energy of 2.80 eV; it was not present in chi(xzx)((2))(omega=2 omega(o)). The enhancement, which may result from a three-photon process involving a midgap defect state, was independent of the carrier concentration, intentional doping, and the presence of the "yellow luminescence band" defects. In addition, we determined sample miscuts by rotational SHG; the miscuts did not generate observable strain induced nonlinearities. The linear optical properties of GaN from 1.38 to 3.35 eV were also determined. [S0163-1829(99)03204-X].