Observation of guided longitudinal acoustic modes in hard supported layers

We report experimental as well as theoretical evidence for the occurrence of a high-frequency acoustic excitation, identified as a longitudinal guided mode, in elastically hard films supported on a soft substrate. While existence criteria related to the transverse and longitudinal sound Velocities in the film and substrate argue for strong decay channels into the substrate, the excitation is evident as a clear peak in the Brillouin light scattering spectra. A Green's function formalism accounts for the existence of this excitation and provides insight into its properties as illustrated for diamond-like-carbon layers deposited on silicon. [S0163-1829(99)50832-1].