Generation of focused surface waves with a solid wedge

Abstract

Focusing of surface acoustic waves on material surfaces was achieved by axicons in liquid immersion. The system consisted of a plane acoustic wave generator obliquely insonifying a cylindrical mirror placed perpendicularly on the surface to be examined. A similar axicon can be implemented with a solid cylindrical structure which does not require an immersion liquid. The lower cap of a solid cylinder is placed on the surface of the solid object, while the upper cap, where a shear-wave transducer is placed, is inclined with respect to the axis of the cylinder. Shear waves generated by the transducer are reflected from the free cylindrical surface of the solid. The reflected waves are incident on the bottom surface at a critical angle and couple to the object surface as surface waves to converge to a diffraction limited focus. The cylindrical solid material must be chosen from low-shear-wave-velocity materials. The critical angle determines the inclination of the top surface; it is found from the solution of reflection problem for shear waves at a solid-solid interface with a slippery boundary condition. The focusing system is implemented successfully with lucite as the solid and aluminum as the object material.