Superscattering and Directive Antennas via Mode Superposition in Subwavelength Core-Shell Meta-Atoms

Alexander W Powell,Alastair P Hibbins,Michal Mrnka,J Roy Sambles

doi:10.3390/photonics9010006

Alexander W Powell, Alastair P Hibbins + Show 2 more

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https://doi.org/10.3390/photonics9010006

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Journal: Photonics	Publication Date: Dec 22, 2021
Citations: 6	License type: CC BY 4.0

Affiliation: University of Exeter

Abstract

Designing a subwavelength structure with multiple degenerate resonances at the same frequency can vastly enhance its interaction with electromagnetic radiation, as well as define its directivity. In this work we demonstrate that such mode superposition or ‘stacking’ can be readily achieved through the careful structuring of a high-permittivity spherical shell, with either a metallic or a low permittivity dielectric (air) core. We examine the behaviour of these structures both as scatterers of plane wave radiation and as directive antennas. In the case where the core is metallic this leads to a superposition of the magnetic and electric modes of the same order, causing suppression of backscattering and unidirectional antenna emission. For an air core, an electric mode can superimpose with the next-highest order magnetic mode, the backscattered power is maximized and antenna emission is bidirectional. This is shown experimentally at microwave frequencies by observing the backscattering of core-shell spheres and we propose two antenna designs demonstrating different emission patterns defined by the superposition of multiple modes.

Highlights

The resonant modes of a subwavelength structure define its interaction with electromagnetic radiation and are fundamental to the operation of many types of antennas, sensors, and metamaterials
In this study we show the versatility of a simple, high symmetry geometry—a two layer sphere for utilizing mode stacking to control microwave emission when resonances are driven either by a plane wave or by an antenna feed
Either an air-filled spherical cavity or a spherical metal ball is defined at the centre of the sphere, and its monostatic radar cross section (RCS) and scattering cross section are calculated in the far field as the radius of this core is changed from 0–6.5 mm

Summary

Introduction

The resonant modes of a subwavelength structure define its interaction with electromagnetic radiation and are fundamental to the operation of many types of antennas, sensors, and metamaterials. Liu et al [8] later proposed using a metal-dielectric core-shell spherical geometry to superimpose both electric and magnetic modes. This proved difficult to replicate experimentally, and the first demonstrations of superscattering of a plane wave via the superposition of higher order resonances have only been published recently by Qian et al [1], and the authors [16]

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