Acoustic Chirality

TL;DR

This paper uncovers a new symmetry and conservation law in linear isotropic elasticity, linking acoustic chirality to phonon populations and introducing concepts like acoustic helicity and false chirality.

Contribution

It establishes chirality as a fundamental property of elastic waves and develops a theoretical framework for chiral acoustic phenomena.

Findings

Integral chirality depends on phonon population imbalance.

Local chirality density involves both transverse and longitudinal waves.

Interference fields show distinct chirality and spin angular momentum distributions.

Abstract

We reveal a previously unknown continuous symmetry and conservation law in the equations of linear isotropic elasticity, which describe the chirality of elastic waves. We show that the integral chirality is determined by the population imbalance between right- and left-handed transverse phonons, whereas the local chirality density generally involves both transverse and longitudinal wave components. We also introduce the related concepts of acoustic helicity and ``false chirality''. The theory is illustrated with simple interference fields exhibiting distinct distributions of chirality, spin angular momentum, and false chirality. Our results establish chirality as a fundamental property of elastic waves and provide a general theoretical framework for chiral acoustic phenomena.