Acoustic flux imaging in anisotropic media

Abstract

A new method for characterizing acoustic flux propagation in anisotropic media is introduced and developed. The technique, which we call ultrasonic flux imaging (UFI), utilizes a pair of water-immersion focused acoustic transducers as a point source and point detector. Raster scanning one transducer produces a transmission pattern which exhibits the anisotropies in acoustic flux known as “phonon focusing” modulated by interference between sheets of the acoustic wave surface. This “internal diffraction” is studied theoretically taking into account the anisotropy of the medium, the boundary conditions between the solid and the water, and the pressure fields produced by the transducers. In addition to bulk effects, the images reveal interesting critical-cone structures associated with the water/solid interface. The theoretical predictions agree well with experimental observations in silicon and a number of other materials, including single-crystal metals, insulators, and semiconductors. All measurements are made at room temperature, in contrast to the cryogenic requirements of previous phononimaging techniques. As a new method, UFI holds promise for examining anisotropies in the vibrational properties, and, possibly, electron-phonon coupling in metals and superconductors. The principles and techniques may also have application to non-destructive characterization of textured polycrystalline and composite materials.