High‐Q Transparency Band in All‐Dielectric Metasurfaces Induced by a Quasi Bound State in the Continuum

Abstract

Bound states in the continuum (BICs) emerge throughout physics as leaky/resonant modes that remain, however, highly localized. They have attracted much attention in optics and photonics, and especially in metasurfaces, that is, planar arrays of sub‐wavelength meta‐atoms. One of their most outstanding features is the arbitrarily large Q‐factors they induce upon approaching the BIC condition, which is exploited here to achieve a narrow transparency band. It is first shown how to shift a canonical BIC in an all‐dielectric metasurface, consisting of high‐refractive disks exhibiting in‐ and out‐of‐plane magnetic dipole (MD) resonances, by tuning the periodicity of the array. By means of the coupled electric/magnetic dipole formulation, it is shown analytically that when the quasi‐BIC overlaps with the broad (in‐plane MD) resonance, a full transparency band emerges with diverging Q‐factor upon approaching the BIC condition in parameter space. Finally, the experimental measurements in the microwave regime with a large array of high‐refractive‐index disks confirm the theoretical predictions. The results reveal a simple mechanism to engineer an ultra‐narrow BIC‐induced transparency band that can be exploited throughout the electromagnetic spectrum with obvious applications in filtering and sensing.

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

“…Our formulation can be exploited to investigate BICs and any other guided/leaky modes supported by metasurfaces and metagratings of interest throughout the electromagnetic spectrum, as long as the predominant optical response of the meta-atoms is of electric and/or magnetic dipolar nature, which is the case of many plasmonic, all-dielectric, and/or hybrid sub-wavelength scatterers. In this regard, recall that our coupled electric/magnetic dipole model for far-field illumination has proven very accurate, not only qualitatively but even quantitatively, in describing the properties of a variety of metasurfaces in the microwave, THz, and visible domains [ 46 , 47 , 48 , 59 , 60 ]. Moreover, cutting-edge experiments for near-field excitation based on the model presented here are indeed underway.…”

“…Among the rich phenomenology they exhibit, metasurfaces have been shown to be a suitable platform to support bound states in the continuum (BICs) with diverging Q factors [ 36 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 ]. The infinitely long (theoretical) lifetimes of such exotic states have been exploited to enhance sensing [ 50 , 51 , 52 , 53 ], filtering [ 54 ], lasing [ 55 , 56 , 57 , 58 , 59 ], electromagnetically-induced transparency [ 60 ], chirality [ 61 ], and non-linearities [ 62 , 63 , 64 ], thus holding promise of unprecedented planar devices in nanophotonics.…”

Analogue of Electromagnetically Induced Transparency in an All-Dielectric Double-Layer Metasurface Based on Bound States in the Continuum

“…If two resonances pass each other as a function of a continuous parameter, then interference causes an avoided crossing of the resonance positions and at one particular set of the parameters, one resonance has an exactly vanishing width and, hence, becomes an F-W BIC [35]. Recently, a high-Q factor EIT was realized based on an S-P quasi-BIC in a dielectric MM in the case of oblique incidence [41]; however, there are no reports on an ideal BICbased EIT. Because the commonly used S-P BIC is a longitudinal dipole BIC, for an ideal S-P BIC, it cannot be coupled with a low-Q transverse dipole resonance in the case of normal incidence [41].…”

“…Recently, a high-Q factor EIT was realized based on an S-P quasi-BIC in a dielectric MM in the case of oblique incidence [41]; however, there are no reports on an ideal BICbased EIT. Because the commonly used S-P BIC is a longitudinal dipole BIC, for an ideal S-P BIC, it cannot be coupled with a low-Q transverse dipole resonance in the case of normal incidence [41]. On the contrary, the F-W BIC in MMs is usually a transverse dipole BIC [36][37][38], which is easy to be coupled with a low-Q transverse dipole resonance, but it is very sensitive to the structural parameters of MMs.…”

All‐Dielectric Toroidal Metasurfaces for Angular‐Dependent Resonant Polarization Beam Splitting

“…In fact, the mode is a bound state in the continuum (BIC), [58,59] which is symmetryprotected at normal incidence, but becomes quasi-BIC with a finite lifetime and hence excited by obliquely incident plane waves. [60,61] A brief analysis of its properties is provided in Section S6, Supporting Information.…”