Asymmetric Metasurfaces with High- Q Resonances Governed by Bound States in the Continuum

Abstract

We reveal that metasurfaces created by seemingly different lattices of (dielectric or metallic) meta-atoms with broken in-plane symmetry can support sharp high-Q resonances that originate from the physics of bound states in the continuum. We prove rigorously a direct link between the bound states in the continuum and the Fano resonances, and develop a general theory of such metasurfaces, suggesting the way for smart engineering of resonances for many applications in nanophotonics and meta-optics.Metasurfaces have attracted a lot of attention in the recent years due to novel ways for wavefront control, advanced light focusing, and ultra-thin optical elements [1]. Recently, metasurfaces based on high-index resonant dielectric materials [2] have emerged as essential building blocks for various functional meta-optics devices [3] due to their low intrinsic loss, with unique capabilities for controlling the propagation and localization of light. A key concept underlying the specific functionalities of many metasurfaces is the use of constituent elements with spatially varying optical properties and optical response characterized by high quality factors (Q factors) of the resonances.Many interesting phenomena have been shown for metasurfaces composed of arrays of meta-atoms with broken inplane inversion symmetry (see Fig. 1), which all demonstrate the excitation of high-Q resonances for the normal incidence of light. The examples are the demonstration of imagingbased molecular barcoding with pixelated dielectric metasurfaces [4] and manifestation of polarization-induced chirality in metamaterials [5], which both are composed of asymmetric pairs of tilted bars [see Fig. 1(a)], observation of trapped modes in arrays of dielectric nanodisks with asymmetric holes [6] [see Fig. 1(b)], sharp trapped-mode resonances in plasmonic and dielectric split-ring structures [7,8] [see, e.g., Fig. 1(c)], broken-symmetry Fano metasurfaces for enhanced nonlinear effects [9, 10] [see Fig. 1(d)], tunable high-Q Fano resonances in plasmonic metasurfaces [11] [see Fig. 1(e)], trapped light and metamaterial-induced transparency in arrays of square split-ring resonators [12,13] presented in Fig. 1(f). Here, we demonstrate that all such seemingly different structures can be unified by a general concept of bound states in the continuum, and we prove rigorously their link to the Fano resonances.Bound states in the continuum (BICs) originated from quantum mechanics as a curiosity [14], but later they were rediscovered as an important physical concept of destructive interference [15] being then extended to other fields of wave physics, including acoustics [16] and optics [17,18]. A true BIC is a mathematical object with an infinite Q factor and vanishing resonance width, it can exist only in ideal lossless infinite structures or for extreme values of parameters [19][20][21]. In practice, BIC can be realized as a quasi-BIC in the form of a supercavity mode [22] when both Q factor and resonance width become finite at the BIC conditions due to ab-d c a...

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

“…This is illustrated by Figure b for normal illumination of PEC ASRR arrays, where the gaps are displaced from the resonator's central vertical axis by the asymmetry parameter d = 0.25 µm (red dashed lines). Both BIC perturbations, either a small illumination angle or a small structural asymmetry, yield QBIC at almost identical spectral positions (Figure b) and similar behavior has also been reported by other groups . Figure also illustrates that the QBIC can be tuned across the broad continuum by changing the coupling distance between the components of the metamaterial unit cell, from high frequencies to an EIT‐like spectrum and low frequencies.…”

“…Both BIC perturbations, either a small illumination angle or a small structural asymmetry, yield QBIC at almost identical spectral positions (Figure 5b) and similar behavior has also been reported by other groups. [22][23][24] Figure 5 also illustrates that the QBIC can be tuned across the broad continuum by changing the coupling distance between the components of the metamaterial unit cell, from high frequencies to an EIT-like spectrum and low frequencies.…”

“…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.…”

Guided‐Mode Resonances in All‐Dielectric Terahertz Metasurfaces

“…Compared to metasurface with W = 180 µm, the one with W = 210 µm open a small band gap due to enhanced magnetic coupling (same reason for the additional resonance) along y-axis. [43] Hence, GMR is inherently a symmetry-protected BIC system at normal incidence and the trapped modes become leaky, i.e., finite Q-factor, and the enhancement of field localization reduces as the angle of incidences goes off-normal. To further analyze the symmetry-protected BICs, we also calculated the Q-factor and average field enhancement 〈E〉/|E 0 | for both metasurfaces, where E 0 is the incident field amplitude and E V ∫ 〈 〉 = 1 | ( )| d E E r r r r is the averaged electric field over the resonator volume, E(r) is the field distribution in 3D coordinates r = (x, y, z) .…”

Ridge Resonance in Silicon Photonics Harnessing Bound States in the Continuum

“…Proposed in 1929 by Neumann and Wigner, these unconventional states can be applied to a large variety of systems and have since been used to describe effects in electromagnetics, electronics, and even acoustics . Many recent demonstrations have shown bound states in the continuum for dielectric metasurfaces . Here relatively weak resonances with a quality factor on the order of 2000 are deemed of interest with applications demonstrated in biosensing and beam control among others.…”