Single-Crystal Folded-Path Delay Lines Utilizing Ultrasonic Pure Shear Modes

Abstract

We describe the principle of folded-path single-crystal delay lines for pure shear modes, utilizing the fact that in most crystal classes there exist special planes with the following properties: (1) A pure shear wave with polarization normal to such a plane can propagate along any direction in it. (2) Upon reflection of this wave on a boundary normal to the plane, no mode conversion occurs. Hence, in a crystal with two or more boundary surfaces normal to such a special plane, a pure shear wave can be reflected back and fourth between these boundaries without reflection loss. Moreover, with an entire plane of propagation directions available, the design can be optimized by selecting the path giving either the largest delay, or, if the attenuation for all propagation directions is known, giving the lowest loss per unit delay. Examples for the propagation and reflection of pure shear modes in cubic and rhombohedral crystals are presented.