Nonlinear electroreflectance from gallium nitride using optical second-harmonic generation

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 ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ nonlinearity but comparable to the linear electroreflectance response (${\mathrm{\omega }}_{\mathrm{in}}$=${\mathrm{\omega }}_{\mathrm{out}}$) in the band-edge region. The nonlinear results from GaN demonstrate the potential of the 2ω 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, ${\mathrm{\chi }}_{\mathit{zyyz}}^{\mathrm{}\left(3\right)\mathrm{}}$(-2ω;ω,ω,0), to be 5.3×${10}^{\mathrm{-}19}$ ${\mathrm{m}}^2$/${\mathrm{V}}^2$. © 1996 The American Physical Society.