RESONANT NONLINEAR SUSCEPTIBILITY NEAR THE GAAS BAND-GAP

We report new results on subpicosecond optical rectification near the semiconductor GaAs band gap at room temperature. We discriminate between radiation produced by real carriers and that produced by optical rectification by using normal and off-normal incident optical illumination. Under normal incident optical illumination of the [111] GaAs surface, the amplitude of the nonlinear susceptibility varies dramatically (2 orders of magnitude) and the sign of the susceptibility reverses when the photon energy is tuned above the band gap. This resonance enhancement and the sign reversal are explained by the dispersion of the second-order nonlinear susceptibility at the band gap.