Theory of the elasto-optic coupling for surface Brillouin scattering in a supported bilayer

We derive explicit expressions of the electromagnetic scattered field and Brillouin cross section for the elasto-optic coupling in a supported bilayer. We consider p-p light scattering and acoustic sagittal modes. The theory is applied to the system Si/SiO2 on Si(001), whose properties are investigated versus the thickness of the Si and SiO2 films. The properties studied are the dispersion relations of surface acoustic modes and pseudomodes, the elasto-optic cross section, and the displacement field profile. A pseudomode of longitudinal character, which is mainly localized in the buried SiO2 film and whose velocity changes monotonically from the longitudinal velocity of Si to that of SiO2, deserves particular interest. © 1995 The American Physical Society.