Symmetry-protected bound states in the continuum (BICs) are nonradiative states with infinite lifetime and perfect confinement of energy even though lying in the radiation continuum due to the symmetry incompatibility. Herein, we study the symmetry-protected BIC supported by metasurfaces composed of silicon nanodisks. Through adding or removing parts of the nanodisks from the edge, a sharp Fano resonance emerges and demonstrates the excitation of quasi-BIC. Their $Q$ factors exhibit the same dependence on the asymmetry degree with these two opposite operations. Furthermore, from both qualitative and quantitative perspectives, analysis on far-field contributions from multipole moments along different directions combining with near-field distributions explains the evolution from BIC to quasi-BIC. The dominant contributor to the quasi-BIC is illustrated to be the electric quadrupole in the $x-y$ plane. Finally, the topological charge carried by the BIC is calculated to be $-1$, demonstrating the topological characteristics of our design. Such metasurfaces are robust in nanofabrication. Our results may provide a route for resonators with better performance applied in sensing, switching, nonlinear optics, and so on.

Enhancing the light-matter interaction in two-dimensional (2D) materials with high $Q$ resonances in photonic structures has boosted the development of optical and photonic devices. Herein we intend to build a bridge between the radiation engineering and the bound states in the continuum (BIC), and present a general method to control light absorption at critical coupling through the quasi-BIC resonance. In a single-mode, two-port system composed of graphene, coupled with silicon nanodisk metasurfaces, the maximum absorption of 0.5 can be achieved when the radiation rate of the magnetic dipole resonance equals to the dissipate loss rate of graphene. Furthermore, the absorption bandwidth can be adjusted more than two orders of magnitude, from 0.9 nm to 94 nm, by simultaneously changing the asymmetric parameter of metasurfaces, the Fermi level, and the layer number of graphene. This work reveals the essential role of BIC in radiation engineering and provides promising strategies in controlling light absorption of 2D materials for the next-generation optical and photonic devices, e.g., light emitters, detectors, modulators, and sensors.

Symmetry‐protected bound states in the continuum (SP‐BICs) are one of the most intensively studied BICs. Typically, SP‐BICs must be converted into quasi‐BICs (QBICs) by breaking the unit cell’s symmetry so that they can be accessed by the external excitation. The symmetry‐broken usually results in a varied resonance wavelength of QBICs which are also highly sensitive to the asymmetry parameters. In this work, we demonstrate that QBICs with a stable resonance wavelength can be realized by breaking translational symmetry in an all‐dielectric metasurface. The unit cell of metasurface is made of a silicon nanodisk dimer. The Q‐factor of QBICs is precisely tuned by changing the interspacing of two nanodisks while their resonance wavelength is quite stable against the interspacing. We also find that such BICs show weak dependence on the shape of the nanodisk. Multiple decompositions indicate that the toroidal dipole dominates this type of QBIC. The resonance wavelengths of QBICs can be tuned only by changing either the lattice constants or the radius of nanodisk. Finally, we present experimental demonstrations on such a QBIC with a stable resonance wavelength. The highest measured Q‐factor of QBICs is >3000. Our results may find promising applications in enhancing light–matter interaction.

The toroidal dipole is a fundamental electromagnetic excitation independent from the well-known electric and magnetic dipoles, which enables strong resonances in nanostructure systems due to the reduction of radiative loss. In this paper, we demonstrate multiple toroidal dipole resonances in a series of silicon nanohole structures with symmetry breaking in the near-infrared range. Detailed multipole decompositions and field distributions are performed to discuss the excitation of these resonances. The following experimental verifications support the theoretical predictions. Such a two-dimensional nanohole platform could be more technically available compared to other complicated nanostructures. Our results may provide remarkable potentials in facilitating the light-matter interaction at the nanoscale, such as light emission, subtle sensing, and so on.

Fano resonance has attracted great attention in nanophotonics attributed to its unique properties. In this work, we study the imaging function of a Fano-resonant silicon metasurface governed by quasi--bound states in the continuum (quasi-BICs). First, by breaking the in-plane symmetry of nanodisks, a symmetry-protected quasi-BIC is excited with the emergence of a sharp Fano resonance. The near-field distributions, multipole contributions, and radiation patterns of the metasurface are investigated to uncover the mechanism and characteristics of this resonance. In addition, we investigate the imaging function of this Fano-resonant metasurface assisted by phase-change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST). Through selective modification of different units from $a$-GST to $c$-GST, the produced transmitted image well reconstructs the target letter. Our findings may provide a route to achieve efficient metasurface-based imaging and fast spatial modulations.