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 30 GHz 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.