Beating resonance patterns and extreme power flux skewing in anisotropic elastic plates

TL;DR

This paper investigates anisotropic elastic wave behavior in silicon plates, revealing orthogonal power flux, zero-group-velocity modes, and resonance patterns caused by slow waves, through laser-ultrasonic experiments.

Contribution

It demonstrates the existence of orthogonal power flux and zero-group-velocity modes in anisotropic plates, providing new insights into wave propagation and resonance phenomena.

Findings

Identification of waves with power flux orthogonal to wave vector

Discovery of zero-group-velocity modes along principal axes

Observation of resonance patterns with moving nodal curves

Abstract

Elastic waves in anisotropic media can exhibit a power flux that is not collinear with the wave vector. This has notable consequences for waves guided in a plate. Through laser-ultrasonic experiments, we evidence remarkable phenomena due to slow waves in a single crystal silicon wafer. Waves exhibiting power flux orthogonal to their wave vector are identified. A pulsed line source that excites these waves reveals a wave packet radiated parallel to the line. Furthermore, there exist precisely eight plane waves with zero power flux. These so-called zero-group-velocity modes are oriented along the crystal's principal axes. Time acts as a filter in the wave vector domain that selects these modes. Thus, a point source leads to beating resonance patterns with moving nodal curves on the surface of the infinite plate. We observe this pattern as it emerges naturally after a pulsed excitation.