Localization of the surface acoustic longitudinal pseudomode of silicon in a buriedSiO2layer

We have measured the Brillouin spectrum in p-p polarization of a bilayer of Si/${\mathrm{SiO}}_2$ produced by low-dose oxygen implantation on the Si (001) surface. The spectra show two strong resonances close to the longitudinal threshold of silicon, which are explained by a full calculation of the Brillouin cross section. In particular the resonance below the threshold is a guided mode of the buried layer (${\mathrm{SiO}}_2$), which originates from the localization in the layer of the surface longitudinal resonance of the matrix (Si). The theory takes into account in a supported double layer both the ripple and the elasto-optic scattering mechanisms.