Bound states in the continuum and Fano resonances in the strong mode coupling regime

Abstract

The study of resonant dielectric nanostructures with high refractive index is a new research direction in nanoscale optics and metamaterial-inspired nanophotonics. Because of the unique opticallyinduced 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. Here we study strong coupling between modes of a single subwavelength high-index dielectric resonator and analyse the mode transformation and Fano resonances when 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 drastic change of 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 a 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 factor helps to extend substantially functionalities of all-dielectric nanophotonics that opens new horizons for active and passive nanoscale metadevices. arXiv:1805.09265v2 [physics.optics] 1 Dec 2018

Nonradiating photonics with resonant dielectric nanostructures

“…In the very recent pioneering work [7], Rybin and co-authors proposed a novel class of high-Q nanoresonators realised by tuning the structure parameters into the socalled supercavity regime [39,82], which is based on the destructive interference of several leaky modes according to the Friedrich-Wintgen mechanism. Later, strong mode coupling regime at the quasi-BIC conditions was observed experimentally in microwaves [83]. Importance of quasi-BICs for meta-optics and nanophotonics, especially, for boost of nonlinear device performance was summarized in the recent paper [84], while we overview a broader scope of applications further.…”

Lattice‐Enhanced Fano Resonances from Bound States in the Continuum Metasurfaces

“…BICs are bound eigenmodes that lie above the photonics light line . This itself is a unique counterintuitive characteristic and because of this, they possess infinite lifetimes and zero‐linewidth resonances under ideal conditions . However, in the real world, ideal conditions are not possible, and therefore the investigation of a quasi‐BIC (QBIC) is the only way to study BIC phenomena.…”

Polarization-based branch selection of bound states in the continuum in dielectric waveguide modes anti-crossed by a metal grating

“…In this case, a mode anti-crossing occurs, and the BIC appears on one of the two split-branches. Such a type of BIC is reported in cylindrical resonator system [37][38][39] and the periodic structures such as photonic [12,28,29,31] or plasmonic structures [33]. As is pointed out by Friedrich and Wintgen [40], the regime where the Q factor is kept high around the BIC becomes larger as the near-field coupling becomes stronger.…”