Surface and pseudosurface acoustic waves in superlattices

We study theoretically the propagation of surface acoustic waves on the surface of a semi-infinite, periodic superlattice consisting of anisotropic, elastic layers of cubic symmetry. By the use of a transfer-matrix method the dispersion relations of surface modes predominantly polarized in the sagittal plane and also those polarized horizontally, are derived. Pseudosurface-wave (PSW) branches are also found inside the frequency bands of. bulk acoustic waves. A remarkable feature is the existence of a PSW branch, which links a surface wave branch below the bulk bands to a branch inside a frequency gap. We present numerical examples for AlAs/GaAs superlattices with an AlAs top surface or a GaAs top surface. Focusing of ballistically propagating surface and pseudosurface waves is also studied.