Polarization-dependent mode coupling in hyperbolic nanospheres

TL;DR

This paper provides a comprehensive theoretical and numerical analysis of hyperbolic nanospheres, revealing their complex modal structures and polarization-dependent responses, which could enable new regimes of light-matter interaction.

Contribution

It introduces a detailed modal analysis of hyperbolic nanospheres, deriving resonance conditions and explaining the origin of magnetic modes due to hyperbolic dispersion.

Findings

Hyperbolic nanospheres exhibit diverse modal responses depending on polarization.

Magnetic dipole modes originate from hyperbolic dispersion and permittivity components.

Strong scattering and absorption are linked to electric-magnetic multipole coupling.

Abstract

Hyperbolic materials offer a much wider freedom in designing optical properties of nanostructures than ones with isotropic and elliptical dispersion, both metallic or dielectric. Here, we present a detailed theoretical and numerical study of the unique optical properties of spherical nanoantennas composed of such materials. Hyperbolic nanospheres exhibit a rich modal structure that, depending on the polarization and direction of incident light, can exhibit either a full plasmonic-like response with multiple electric resonances, a single, dominant electric dipole or one with mixed magnetic and electric modes with an atypical reversed modal order. We derive resonance conditions for observing these resonances in the dipolar approximation and offer insight into how the modal response evolves with the size, material composition, and illumination. Specifically, the origin of the magnetic dipole mode lies in the hyperbolic dispersion and its existence is determined by two diagonal permittivity components of different sign. Our analysis shows that the origin of this unusual behavior stems from complex coupling between electric and magnetic multipoles, which leads to very strongly scattering or absorbing modes. These observations assert that hyperbolic nanoantennas offer a promising route towards novel light-matter interaction regimes.