NONLINEAR ELECTROREFLECTANCE FROM GALLIUM NITRIDE USING OPTICAL 2ND-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 absorpti on edge, a strong two-photon resonance was observed in the reflected S H signal when a surface de electric field in the range of 110-575 kV/c m was applied to the GaN/electrolyte interface. The resonance was attr ibuted to electric-field-induced SH (EFISH) generation, a third-order nonlinear response that generates a signal intensity that is quadradic ally dependent on the de field in the film. The width of the EFISH res onance at the E(0) critical point was much narrower than the dispersio n of the intrinsic chi((2)) nonlinearity but comparable to the linear electroreflectance response (omega(in) = omega(out)) in the band-edge region. The nonlinear results from GaN demonstrate the potential of th e 2 omega response for spectroscopic examination of critical points in the band structure of semiconductors. In addition to the spectral ana lysis, a fixed-frequency EFISH measurement at the peak of the resonanc e (3.43 eV) determined the magnitude of the third-order nonlinearity, chi(zyyz)((3))(-2 omega;omega,omega,0), to be 5.3 x 10(-19) m(2)/V-2.