Acoustic vibrations in free-standing double layer membranes

We report on a Brillouin light scattering study of acoustic excitations in free-standing polymethyl methacrylate (PMMA)∕Si₃N₄ double layer membranes. For vanishing-wave vector transfer along the membrane, the observed excitations are a series of longitudinal standing wave harmonics whose frequencies depend on PMMA and Si₃N₄ layer thickness. The associated displacement profiles below 300.3em0exGHz are largely confined to the softer PMMA layer with mode amplitudes significantly higher at the PMMA surface than at the free surface of the high elastic modulus Si₃N₄ layer. At higher frequencies, in the vicinity and beyond the resonance associated with the Si₃N₄ layer, the mode amplitudes become comparable at both free surfaces. Excitations with transverse polarization as well as waves whose amplitudes within the Si₃N₄ are consistent with flexural and dilatational modes for a single layer are also observed. For certain ranges of wave vector the latter interact and hybridize with the harmonics arising from the PMMA layer.