Line-focus probe excitation of Scholte acoustic waves at the liquid-loadedsurfaces of periodic structures

A model is introduced to explain our observation of Scholte-like ultrasonic waves traveling at the water-loaded surfaces of solids with periodically v arying properties. The observations pertain to two two-dimensional superlat tices: a laminated solid of alternating 0.5-mm-thick layers of aluminum and a polymer, and a hexagonal array of polymer rods of lattice spacing 1 mm i n an aluminum matrix. The surface waves are generated and detected by line focus acoustic lenses aligned parallel to each other, and separated by vary ing distances. The acoustic fields of these lenses may be considered a supe rposition of plain bull; waves with wave normals contained within the angul ar apertures of the lenses. For homogeneous solids, phase matching constrai nts do not allow the Scholte wave to be coupled into with an experimental c onfiguration of this type. This is not true for a spatially periodic solid, where coupling between bulk waves and the Scholte surface wave takes place through Umklapp processes involving a change in the wave-vector component parallel to the surface by a reciprocal lattice vector. In the experiments, the source pulse is broadband, extending up to about 6 MHz, whereas the sp ectrum of the observed Scholte wave is peaked at around 4 and 4.5 MHz for t he layered solid and hexagonal lattice, respectively. We attribute this to a resonance in the surface response of the solid, possibly associated with a critical point in the dispersion relation of the superlattice. On rotatin g the solid about its surface normal, the Scholte wave displays dramatic va riation in phase arrival time and, to a lesser extent, also group arrival t ime. This variation is well accounted for by our model. [S0163-1829(99)0053 9-1].