Non-Mie Optical Resonances in Anisotropic Biomineral Nanoparticles

TL;DR

This paper introduces a new class of optical resonances in low-index, anisotropic biomineral nanoparticles that do not follow traditional Mie resonance behavior, expanding the understanding of nanophotonic responses.

Contribution

It demonstrates the existence of anisotropy-induced non-Mie eigenmodes in biomineral nanoparticles, providing new insights into their optical scattering properties and resonances.

Findings

Anisotropic biomineral nanoparticles exhibit unique scattering peaks.

These peaks correspond to a new family of electric and magnetic resonances.

Biominerals like calcite and vaterite show distinct optical responses due to internal structure.

Abstract

Optical properties of nanoparticles attract continuous attention owing to their high fundamental and applied importance across many disciplines. Recently emerged field of all-dielectric nanophotonics employs optically induced electric and magnetic Mie resonances in dielectric nanoparticles with a high refractive index. This property allows obtaining additional valuable degrees of freedom to control optical responses of nanophotonic structures. Here we propose a conceptually distinct approach towards reaching optical resonances in dielectric nanoparticles. We show that lacking conventional Mie resonances, low-index nanoparticles can exhibit a novel anisotropy-induced family of non-Mie eigenmodes. Specifically, we investigate light interactions with calcite and vaterite nanospheres and compare them with the Mie scattering by a fused silica sphere. Having close permittivities and same dimensions, these particles exhibit significantly different scattering behavior owing to their internal structure. While a fused silica sphere does not demonstrate any spectral features, the uniaxial structure of the permittivity tensor for calcite and non-diagonal permittivity tensor for vaterite result in a set of distinguishable peaks in scattering spectra. Multipole decomposition and eigenmode analysis reveal that these peaks are associated with a new family of electric and magnetic resonances. We identify magnetic dipole modes of ordinary, extraordinary and hybrid polarization as well as complex electric dipole resonances, featuring a significant toroidal electric dipole moment. As both vaterite and calcite are biominerals, naturally synthesized and exploited by a variety of living organisms, our results provide an indispensable toolbox for understanding and elucidation of mechanisms behind optical functionalities of true biological systems.