High-frequency standing longitudinal acoustic resonances in supported thin films

Inelastic light scattering from epitaxial ZnSe films grown on GaAs reveal a large number of near equally spaced phonon modes with characteristic frequencies that depend on the film thickness and scale as the harmonics of a closed-end organ pipe. These periodic excitations that extend well beyond 100 GHz are identified as confined longitudinal acoustic resonances with negligible in-plane wave vector components. The calculated scattering cross sections, which consider the elasto-optic and surface/interface ripple channels in the film and substrate, account for all significant features of the Brillouin spectra. These include the conspicuous appearance of standing wave harmonics only beyond {~}45 GHz and apparent absence of Brillouin peaks related to alternate harmonics.