Abstract

Recently, amplified femtosecond Ti:Sapphire lasers enabled the observation of fourth harmonic generation (FHG) from crystalline surfaces of noncentrosymmetric (GaAs(001)) and centrosymmetric (Si(001)) semiconductors below the damage threshold.¹ We now extend this work by observing χ⁽⁴⁾ at the Si(111)/SiO₂ interface, not only by direct FHG, but by electromodulated third harmonic generation (THG), which is proportional to ${\left|{\text{χ}}_{\text{surface}}^{(4)}{E}^3(\omega )E_{\text{dc}}\right|}^2$ The latter provides a much stronger signal at lower fundamental power than FHG. The anisotropic properties of THG and FHG have been measured by azimuthal sample rotation and compared to a phenomenological model based on the symmetry of the (111) surface. THG and FHG have been measured from n-Si(111)/SiO₂/Cr MOS capacitors with varying bias to allow the observation, for the first time, of DC electric field induced higher harmonics.

© 1999 Optical Society of America