Acoustic helical dichroism in a one-dimensional lattice of chiral resonators

TL;DR

This paper demonstrates acoustic helical dichroism in a one-dimensional lattice of chiral resonators, enabling differential absorption of sound vortices with opposite orbital angular momentum, with potential applications in sound manipulation.

Contribution

It introduces the concept of acoustic helical dichroism in chiral resonator lattices and analyzes its dependence on symmetry and non-Hermitian physics.

Findings

Differential absorption of acoustic vortices with opposite OAM observed.

Breaking $C_4$ symmetry induces OAM bandgaps and exceptional points.

Effective Hamiltonian models the complex band structures accurately.

Abstract

Circular dichroism and helical dichroism are intriguing chiroptical phenomena with broad applications in optical sensing and imaging. Here, we generalize one of the phenomena-helical dichroism-to acoustics. We show that a one-dimensional lattice of chiral resonators with loss can induce differential absorption of helical sounds (i.e. acoustic vortices) carrying opposite orbital angular momentum (OAM). This acoustic helical dichroism strongly depends on the rotation symmetry of the chiral resonators. A breaking of the $C_4$ rotation symmetry can induce coupling between the opposite chiral dipole modes of the resonators. This leads to OAM bandgaps and non-Hermitian exceptional points near the Brillouin-zone center and boundaries, which together give rise to significantly enhanced helical dichroism. The underlying physics can be well captured by an effective Hamiltonian that quantitatively reproduces the complex band structures. The acoustic helical dichroism can find important applications in acoustic OAM manipulations and chiral sound-matter interactions.