Probing the surface of silicon and the silicon-silica interface using nonperturbative third and fifth harmonic generation

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.