Multipolar theory of bianisotropic response

TL;DR

This paper extends the theory of bianisotropy in metaatoms beyond dipole interactions to include high-order multipoles, emphasizing the role of structural symmetry in inducing bianisotropic responses.

Contribution

It generalizes bianisotropic response theory to arbitrary multipole orders, linking it solely to the lack of inversion symmetry in metaatoms.

Findings

Bianisotropy is caused by the absence of inversion symmetry.

Higher-order multipoles prevent violations of Onsager-Casimir conditions.

Symmetry, not size, determines bianisotropic strength.

Abstract

Bianisotropy of metaatoms is usually associated with their nonlocal response and the mutual coupling between electric and magnetic dipole moments induced by the incident field. In this work, we generalize the theory of bianisotropy beyond the dipole response to the cases of arbitrary high-order multipole resonances. We demonstrate that bianisotropy is exclusively caused by the absence of the inversion symmetry of metaatoms. The strength of the bianisotropy response is normally increased with the size of a meta-atom but its origin is fully connected to the symmetry of the structure. As an important example of bianisotropic particle, we consider a triangular prism and show how accounting for the higher-order multipoles prevents the violation of the Onsager-Casimir conditions for kinetic coefficients appearing in the dipole approximation. The developed theory is an important step towards a deeper insight into the scattering properties of nanoantennas and meta-atoms.