Acoustic circular dichroism in a three-dimensional chiral metamaterial

TL;DR

This paper demonstrates the first acoustic circular dichroism effect in a three-dimensional chiral metamaterial, showing how symmetry and loss influence the phenomenon and its potential applications in acoustic sensing and manipulation.

Contribution

It introduces the acoustic CD effect in 3D chiral metamaterials and analyzes how symmetry and loss affect the effect, highlighting the role of complex band structure and non-Hermitian phenomena.

Findings

Negligible CD in $C_4$ symmetric, lossy metamaterials.

Enhanced CD in $C_2$-symmetric systems with inhomogeneous loss.

Correlation of CD enhancement with polarization bandgaps and exceptional points.

Abstract

Circular dichroism (CD) is an intriguing chiroptical phenomenon associated with the interaction of chiral structures with circularly polarized lights. Although the CD effect has been extensively studied in optics, it has not yet been demonstrated in acoustic systems. Here, we demonstrate the acoustic CD effect in a three-dimensional chiral metamaterial supporting circularly polarized transverse sound. We find that the effect is negligible in the lossy metamaterial possessing $C_4$ rotational symmetry but can be strongly enhanced in the $C_2$-symmetric system with inhomogeneous loss. The phenomena can be understood based on the properties of the metamaterial's complex band structure and the quality factors of its eigenmodes. We show that the enhanced CD in the $C_2$-symmetric system is attributed to the polarization bandgaps and the non-Hermitian exceptional points appearing near the Brillouin-zone center and boundaries. The results contribute to the understanding of chiral sound-matter interactions and can find applications in acoustic sensing of chiral structures and sound manipulations based on its vector properties.