Interference between the modes of an all-dielectric meta-atom

TL;DR

This paper investigates the modes of silicon meta-atoms, revealing how interference between non-orthogonal modes affects extinction and scattering, with implications for designing directional nano-photonic devices.

Contribution

It introduces a robust quasi-normal mode approach to analyze interference effects in dielectric meta-atoms, enhancing understanding of their scattering properties.

Findings

Interference between non-orthogonal modes can cause negative contributions to extinction.

Matching electric and magnetic dipole moments leads to strong forward scattering.

Higher order modes enable multiple directional scattering bands.

Abstract

The modes of silicon meta-atoms are investigated, motivated by their use as building blocks of Huygens' metasurfaces. A model based on these modes is presented, giving a clear physical explanation of all features in the extinction spectrum. Counter-intuitively, this can show negative contributions to extinction, which are shown to arise from the interference between non-orthogonal modes. The direct and interference contributions to extinction are determined, showing that conservation of energy is preserved. The Huygens' condition of matched electric and magnetic dipole moments leads to strong forward scattering and suppressed back scattering. It is shown that higher order modes with appropriate symmetry generalise this condition, leading to multiple bands of directional scattering. The presented results are obtained using a robust approach to find the modes of nano-photonic scatterers, commonly referred to as quasi-normal modes. By utilising an integral formulation of Maxwell's equations, this work avoids the problem of normalising diverging far-fields, which other approaches require. The model and presented results are implemented in open-source code.