Observation of organ-pipe acoustic excitations in supported thin films - art. no. 081402

Brillouin light scattering from supported silicon oxynitride films reveal a n extended series of acoustic excitations occurring at regular frequency in tervals when the mode wave vector is perpendicular to the film surface. The se periodic peaks are identified as distinct standing wave excitations that , similar to harmonies of an open-ended organ pipe, occur due to the bounda ry conditions imposed by the free surface and substrate-film interface. The surface ripple and volume elasto-optic scattering mechanisms contribute to the scattering cross sections and lead to dramatic interference effects at low frequencies where the surface corrugations play a dominant role. The t ransformation of these standing wave excitations to modes with finite in-pl ane wave vectors is also investigated. The results are discussed in the fra mework of a Green's-function formalism that reproduces the experimental fea tures and illustrate the importance of the standing modes in evaluating the longitudinal elastic properties of the films.