Nonlinear electroreflectance from gallium nitride using optical second-harmonic generation.

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The nonlinear optical properties of an electrified GaN film in contact with an electrolytic solution were investigated using second-harmonic (SH) generation. For SH photon energies near the fundamental absorption edge, a strong two-photon resonance was observed in the reflected SH signal when a surface dc electric field in the range of 110--575 kV/cm was applied to the GaN/electrolyte interface. The resonance was attributed to electric-field-induced SH (EFISH) generation, a third-order nonlinear response that generates a signal intensity that is quadradically dependent on the dc field in the film. The width of the EFISH resonance at the ${\mathit{E}}_{0}$ critical point was much narrower than the dispersion of the intrinsic ${\mathrm{\ensuremath{\chi}}}^{(2)}$ nonlinearity but comparable to the linear electroreflectance response (${\mathrm{\ensuremath{\omega}}}_{\mathrm{in}}$=${\mathrm{\ensuremath{\omega}}}_{\mathrm{out}}$) in the band-edge region. The nonlinear results from GaN demonstrate the potential of the 2\ensuremath{\omega} response for spectroscopic examination of critical points in the band structure of semiconductors. In addition to the spectral analysis, a fixed-frequency EFISH measurement at the peak of the resonance (3.43 eV) determined the magnitude of the third-order nonlinearity, ${\mathrm{\ensuremath{\chi}}}_{\mathit{zyyz}}^{(3)}$(-2\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},0), to be 5.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ ${\mathrm{m}}^{2}$/${\mathrm{V}}^{2}$. \textcopyright{} 1996 The American Physical Society.