# Intrinsic multipolar contents of nanoresonators for tailored scattering

## TL;DR

This paper presents a new method to design nanoresonators with intrinsic, excitation-independent multipolar responses, enabling tailored scattering properties for advanced nanophotonic applications.

## Contribution

The authors develop a theoretical and computational framework to engineer the intrinsic multipolar content of resonators, focusing on their natural quasinormal modes, which is a novel approach.

## Key findings

- Designed a subwavelength optical resonator with Janus resonance.
- Achieved side-dependent coupling over the full resonance linewidth.
- Method applicable to all wave types for scattering and emission control.

## Abstract

We introduce a theoretical and computational method to design resonant objects, such as nanoantennas or meta-atoms, exhibiting tailored multipolar responses. In contrast with common approaches that rely on a multipolar analysis of the scattering response of an object upon specific excitations, we propose to engineer the \textit{intrinsic} (i.e., excitation-independent) multipolar content and spectral characteristics of the natural resonances -- or quasinormal modes -- of the object. A rigorous numerical approach for the multipolar decomposition of resonances at complex frequencies is presented, along with an analytical model conveying a direct physical insight into the multipole moments induced in the resonator. Our design strategy is illustrated by designing a subwavelength optical resonator exhibiting a Janus resonance that provides side-dependent coupling to waveguides over the full linewidth of the resonance and on a wide angular range for linearly-polarized incident planewaves. The method applies to all kinds of waves and may open new perspectives for subwavelength-scale manipulation of scattering and emission.