  We examined, in backward (reflection) geometry, the generation of the 3rd and
5th harmonics, located in the deep and vacuum ultraviolet, on the surface of
silicon and on the interface between silicon and silica when a thin silica film
was grown on a silicon substrate. In both cases, a strong dependence of the
harmonic signal on the polarization direction of the driving laser beam was
found. The differences observed for both samples, are qualitatively explained.
Furthermore, a simplified tensor formalism for the polarization dependence is
introduced, which reveals the structural symmetry of the surface and the
interface and describes the polarization dependence with high accuracy. The
study is an essential step to further understand nonlinear interaction and
nonperturbative harmonic generation on the boundaries of materials.

Optics