Pseudosurface wave structures in phonon imaging

Abstract

A parameter-free model for calculating surface directivity effects in phonon images is described. It treats the coupling between crystal and metal-heater and -detector films as an isotropic dissipative mechanism, and it is applicable to situations where the overlayer is of low acoustic impedance or weakly bonded to the crystal substrate. Our calculated phonon-intensity diagrams display well-defined structures which derive from pseudosurface waves (PSW’s) in the quasifree crystal surface. These waves differ from Rayleigh waves in that they contain a small bulk-wave component through which they can be excited and into which they decay. Some of these PSW’s are closely associated with exit and reentrant critical cones for mode conversion at the crystal surface. In some cases the PSW’s evolve, in limiting directions, into true surface waves immersed in a continuum of bulk waves. Phonon imaging is shown to be a useful technique for studying these complex and interesting resonant vibrations in crystal surfaces, and this enhances the potential value of this technique as a sensitive probe of crystal surfaces and interfaces.