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• Vol. 1, Issue 1, 016001 (2019)
Andrey A. Bogdanov1、2, Kirill L. Koshelev1、3, Polina V. Kapitanova1, Mikhail V. Rybin1、2, Sergey A. Gladyshev1, Zarina F. Sadrieva1, Kirill B. Samusev1、2, Yuri S. Kivshar1、3、*, and Mikhail F. Limonov1、2
Author Affiliations
• 1ITMO University, Department of Nanophotonics and Metamaterials, St. Petersburg, Russia
• 2Ioffe Institute, St. Petersburg, Russia
• 3Australian National University, Nonlinear Physics Center, Canberra, Australia
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Andrey A. Bogdanov, Kirill L. Koshelev, Polina V. Kapitanova, Mikhail V. Rybin, Sergey A. Gladyshev, Zarina F. Sadrieva, Kirill B. Samusev, Yuri S. Kivshar, Mikhail F. Limonov. Bound states in the continuum and Fano resonances in the strong mode coupling regime[J]. Advanced Photonics, 2019, 1(1): 016001 Copy Citation Text show less

Abstract

The study of resonant dielectric nanostructures with a high refractive index is a new research direction in the nanoscale optics and metamaterial-inspired nanophotonics. Because of the unique optically induced electric and magnetic Mie resonances, high-index nanoscale structures are expected to complement or even replace different plasmonic components in a range of potential applications. We study a strong coupling between modes of a single subwavelength high-index dielectric resonator and analyze the mode transformation and Fano resonances when the resonator’s aspect ratio varies. We demonstrate that strong mode coupling results in resonances with high-quality factors, which are related to the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich–Wintgen scenario of destructive interference. We explain the physics of these states in terms of multipole decomposition, and show that their appearance is accompanied by a drastic change in the far-field radiation pattern. We reveal a fundamental link between the formation of the high-quality resonances and peculiarities of the Fano parameter in the scattering cross-section spectra. Our theoretical findings are confirmed by microwave experiments for the scattering of high-index cylindrical resonators with a tunable aspect ratio. The proposed mechanism of the strong mode coupling in single subwavelength high-index resonators accompanied by resonances with high-quality factors helps to extend substantially functionalities of all-dielectric nanophotonics, which opens horizons for active and passive nanoscale metadevices.

1 Introduction

The physics of resonant structures with a strong mode coupling is of fundamental importance, and is responsible for a variety of interesting phenomena governing both transport and localization of waves. The modes supported by traditional resonators and microcavities1 exist due to the reflection of waves from the resonator’s boundaries under the conditions of constructive interference. To achieve high values for the resonator’s quality factor ($Q$-factor), one can improve reflectivity, whether by using metals,2,3 photonic bandgap structures,4 or the total internal reflection at glancing angles of incidence in whispering-gallery-mode (WGM) resonators.5 Such physical mechanisms require large-sized cavities with a complex design. A more attractive way to confine light is to use destructive interference in the regime of strong mode coupling.68 This mechanism is related to the physics of bound states in the continuum (BIC).9 It was first proposed in quantum mechanics by Friedrich and Wintgen10 and then was extended to acoustics1113 and electrodynamics.14,15 A true optical BIC is a mathematical abstraction, as its realization demands either infinite size of the structure or zero (or infinite) permittivity.1618 However, the BIC-inspired mechanism of light localization makes possible realization of high-$Q$ states in photonic crystal cavities and slabs,15,17,19 coupled waveguide arrays,2022 dielectric gratings,14 core–shell spherical particles,18 dielectric resonators,2326 and hybrid plasmonic-photonic systems.27

Andrey A. Bogdanov, Kirill L. Koshelev, Polina V. Kapitanova, Mikhail V. Rybin, Sergey A. Gladyshev, Zarina F. Sadrieva, Kirill B. Samusev, Yuri S. Kivshar, Mikhail F. Limonov. Bound states in the continuum and Fano resonances in the strong mode coupling regime[J]. Advanced Photonics, 2019, 1(1): 016001