Near-Field Excitation of Bound States in the Continuum in All-Dielectric Metasurfaces through a Coupled Electric/Magnetic Dipole Model.

Diego R Abujetas,José A Sánchez-Gil

doi:10.3390/nano11040998

Abstract

Resonant optical modes arising in all-dielectric metasurfaces have attracted much attention in recent years, especially when so-called bound states in the continuum (BICs) with diverging lifetimes are supported. With the aim of studying theoretically the emergence of BICs, we extend a coupled electric and magnetic dipole analytical formulation to deal with the proper metasurface Green function for the infinite lattice. Thereby, we show how to excite metasurface BICs, being able to address their near-field pattern through point-source excitation and their local density of states. We apply this formulation to fully characterize symmetry-protected BICs arising in all-dielectric metasurfaces made of Si nanospheres, revealing their near-field pattern and local density of states, and, thus, the mechanisms precluding their radiation into the continuum. This formulation provides, in turn, an insightful and fast tool to characterize BICs (and any other leaky/guided mode) near fields in all-dielectric (and also plasmonic) metasurfaces, which might be especially useful for the design of planar nanophotonic devices based on such resonant modes.

Highlights

IntroductionCoupled-dipole formulations have been exploited since long ago to describe the optical properties of particles (typically of sub-wavelength dimensions) behaving as point dipoles [1,2]
We extend a previously developed coupled electric and magnetic dipole (CEMD) formulation for plane wave excitation [19], to deal with the proper Green function of the scattering problems
We focus on two symmetry-protected BICs arising at the Γ point from vertical dipole resonances, which exhibit the expected quasi-BIC

Summary

Introduction

Coupled-dipole formulations have been exploited since long ago to describe the optical properties of particles (typically of sub-wavelength dimensions) behaving as point dipoles [1,2]. These formulations allow to address the coupling between such dipolar particles in a variety of arrangements. 2D planar arrays consisting of resonant dipolar/multipolar particles have attracted increased attention [3–20], lately including magnetic dipole resonances to account for the lowest-order Mie resonances of high-refractive index particles [5,21–26]. The renewed interest in such planar arrays stems from their application as infinitely thin optical devices performing various functionalities [27–40], especially in nanophotonics, known as metasurfaces/metagratings in the non-diffractive/diffractive spectral regime, respectively

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Conclusion