Optical Study of Ultrasonic Diffraction and Focusing in Anisotropic Media

Abstract

This paper describes an investigation of the diffraction of 100–500-Mc/sec longitudinal elastic waves in anisotropic materials, using the techniques of optical (Bragg) scattering, which allow direct measurement of ultrasonic intensity distributions along the path of the beam. Such measured distributions for beams propagating along pure mode axes in crystal quartz, silicon, and potassium bromide are shown to agree very well with intensity distributions predicted using a diffraction integral derived from a rigorous solution of the related problem of electromagnetic diffraction in uniaxially anisotropic media. In addition, it is demonstrated that cylindrical crystal surfaces, symmetric about the proper pure-mode axis, are appropriate excitation surfaces for cylindrically focused ultrasonic beams. The focusing properties of thin-film CdS and ZnO transducers evaporated on cylindrical surfaces of crystal quartz and silicon are described and compared with the properties of focused beams in isotropic materials.