We report our theoretical considerations motivated by Brillouin spectra of polymethyl methacrylate (PMMA) films ( d = 13 nm and d = 225 nm) on Si(100) substrates. We have rederived a numerical calculational scheme developed for considering Brillouin experiments on layer systems. It delivers dispersion curves and intensities, indicated here by appropriate hatchings. With backscattering geometry, we observed guided waves in agreement with our calculations. The Brillouin peaks fall on the calculated segments of the dispersion curves visible with the elasto-optic mechanism for 180° scattering or for reflection-induced `A-scattering' (called RIΘA scattering by Krüger et al (Krüger J K, Embs J, Brierley J and Jimenez R 1998 J. Phys. D: Appl. Phys. 31 1913)). The observed pseudo-guided waves for d = 225 nm were explained using a model of a PMMA film fixed at one surface and free at the other. For the d = 225 nm thick film the Brillouin data provided information on its mechanical constants, but in the d = 13 nm case the dispersion curves are not very sensitive to the film properties. The Brillouin data for d = 13 nm show the direction of the sagittal plane with respect to the symmetry of the silicon rather than indicating the properties of the PMMA film.
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Hotz et al. (2001) studied this question.