SURFACE ANISOTROPY IN OPTICAL 2ND-HARMONIC GENERATION .2. EMBEDDING APPROACH TO AL(111) AND VICINAL AL(001) SURFACES

The anisotropic nonlinear optical response of Al(111) and vicinal Al(0 01) surfaces is calculated microscopically with the aim of analyzing s econd harmonic generation measurements on these systems. The electroni c states and currents are evaluated within the embedding approach whic h provides a more accurate description than the two-band model for Al slabs used previously by Petukhov and Liebsch. Although we find apprec iable differences in detail, the qualitative picture derived from thes e calculations is confirmed. In particular, we verify that the anisotr opic current giving rise to the chi(xxx)(omega) component of the nonli near surface polarizability exhibits a remarkably large penetration de pth and a strong frequency dependence close to the main interband tran sitions of Al. For Al(111) as well as vicinal Al surfaces, the physica l origin of the anisotropy is the nonsymmetric stacking of the lattice planes parallel to the macroscopic surface rather than the corrugatio n of the outer electronic density profile.