SURFACE ANISOTROPY IN OPTICAL 2ND-HARMONIC GENERATION .1. AL(111)

A microscopic calculation of the anisotropic nonlinear optical surface response of Al(111) is performed within the nearly free electron appr oximation. The bulk potential is described in terms of the most import ant Fourier components of the lattice potential and the self-consisten t Lang-Kohn potential is used as surface barrier. The dynamical respon se is treated within the independent particle approximation. The aniso tropic tangential surface current is shown to exhibit a surprisingly l arge penetration depth that is determined by the spatial inhomogeneity of the electronic states near the band gaps. The frequency dependence of the anisotropic surface polarizability element chi(xxx)(omega) exh ibits resonant behavior at omega or 2 omega near 1.5 eV, i.e. close to the onset of the main interband transitions caused by V-200. The magn itude of the anisotropic surface polarizability is comparable to that of the main isotropic component. These results are in qualitative agre ement with the available experimental data on Al(111). It is argued th at the large penetration depth of the tangential surface current does not imply a weak surface sensitivity of chi(xxx) since this current sh ould be sensitive to the electron scattering conditions at the surface .