Extreme acoustic anisotropy in crystals visualized by diffraction tensor

TL;DR

This paper introduces a rapid, robust method using diffraction tensor eigenvalues and stereographic projection to visualize and analyze acoustic anisotropy in crystals, aiding device design and orientation optimization.

Contribution

A novel method for survey and visualization of acoustic anisotropy based on diffraction tensor eigenvalues, applicable to various crystals and device design.

Findings

Identified singular directions of BAW propagation in crystals

Visualized areas of fast and slow wave velocity variation

Demonstrated application to lithium niobate for high-velocity surface waves

Abstract

Acoustic wave propagation in single crystals, metamaterials and composite structures is a basic mechanism in acoustic, acousto-electronic and acousto-optic devices. Acoustic anisotropy of crystals provides a variety of device performances and application fields, but its role in pre-estimation of achievable device characteristics and location of crystal orientations with desired properties is often underestimated. A geometrical image of acoustic anisotropy can be an important tool in design of devices based on wave propagation in single crystals or combinations of anisotropic materials. We propose a fast and robust method for survey and visualization of acoustic anisotropy based on calculation of the eigenvalues of bulk acoustic wave (BAW) diffraction tensor (curvature of the slowness surface). The stereographic projection of these eigenvalues clearly reveals singular directions of BAW propagation (acoustic axes) in anisotropic media and areas of fast or slow variation of wave velocities. The method is illustrated by application to three crystals of different symmetry used in different types of acoustic devices: paratellurite, lithium niobate, and potassium gadolinium tungstate. The specific features of acoustic anisotropy are discussed for each crystal in terms of their potential application in devices. In addition, we demonstrate that visualization of acoustic anisotropy of lithium niobate helps to find orientations supporting propagation of high-velocity surface acoustic waves.