Fig. 1. Principle and structure design diagram for the realization of strongly robust high-Q quasi-BICs. (a) A typical SP-BIC. (b) Interconversion between two SP-BICs. (c) Schematic of an all-dielectric metasurface composed of four square blocks. (d) Methods for inducing excitation of quasi-BICs.

Fig. 2. Optical properties of quasi-BICs: MR and TR. (a) Transmission spectra of the asymmetric metasurface (Δs ≠ 0 nm). Dual resonance responses are marked by MR and TR, respectively. (b), (c) Resonance wavelengths and Q-factors of the MR and TR resonances, respectively, versus the asymmetric parameter Δs. (d), (e) Scattered powers of the MR and TR resonances, respectively, and the electromagnetic-field distributions for the MR and TR resonances, respectively.

Fig. 3. Optical properties of quasi-BICs: λ₁-λ₁^′ and λ₂-λ₂^′. (a) Transmission spectra of the asymmetric metasurface (ΔL ≠ 0 nm) at E_y-polarized incidence. The remaining two resonance responses, excluding MR and TR, are denoted as λ₁ and λ₂ when ΔL < 0 nm, and as λ₂^′ and λ₁^′ when ΔL > 0 nm. (b) Distributions of the z-component of the magnetic fields (H_z) in the x–y plane for various ΔL values corresponding to the λ₁, λ₂, λ₁^′, and λ₂^′ resonances. Black arrows indicate the x–y plane electric field vector E_xy. (c) Resonance wavelength and Q-factor of the λ₁-λ₁^′ resonance concerning the asymmetric parameter ΔL (−60 to 60 nm). (d) Resonance wavelength and Q-factor of the λ₂-λ₂^′ resonance concerning the asymmetric parameter ΔL (−350 to 60 nm).

Fig. 4. Electromagnetic-field distributions and demonstration of the role of dipole excitation in quasi-BIC resonances: λ₁^′ and λ₂^′. (a)–(d) Electromagnetic-field distributions of the λ₁^′ (ΔL = 1 nm), λ₁^′ (ΔL = 40 nm), λ₂^′ (ΔL = 1 nm), and λ₂^′ (ΔL = 40 nm) resonances, respectively. Black arrows indicate the displacement current vector and magnetic-field vector, respectively. (e)–(h) Scattered powers of the λ₁^′ (ΔL = 1 nm), λ₁^′ (ΔL = 40 nm), λ₂^′ (ΔL = 1 nm), and λ₂^′ (ΔL = 40 nm) resonances, respectively.

Fig. 5. Variation of the electromagnetic field distributions in the x–y plane within the quasi-BIC resonance mode λ₂ while varying ΔL from −50 to −350 nm.