Fig. 1. (a) Schematic of the proposed S-shaped all-dielectric metasurface structure and unit cell of the metasurface, respectively. (b) Case I: Introduction of geometric asymmetry by varying the length of the vertical silicon bars. (c) Case II: Introduction of geometric asymmetry by altering the length of the horizontal silicon bars. (d) Case III: Introduction of field-asymmetry by adjusting the incident angle θ.

Fig. 2. (a), (d) Intrinsic band structures and (b), (e) Q-factors of TM and TE modes, respectively. (c), (f) Electromagnetic field distributions corresponding to TM and TE modes at Γ point, respectively.

Fig. 3. (a) Simulated transmission spectra of the S-shaped metasurface in the symmetric and asymmetric cases (Δg = 10 nm, Δd = 10 nm, and θ = 3°) at the normally x-polarized incident wave, respectively. (b) Fano-fitted transmission spectra of TM₁, TM₂, and TM₄ in case I (Δg = 10 nm). (c)–(e) Q-factors of the TM₁, TM₂, and TM₄ resonances concerning degrees of asymmetry (Δg, Δd, and θ) for Cases I–III, respectively.

Fig. 4. Scattered powers of the (a) TM₁, (b) TM₂, and (d) TM₄ resonances in case I (Δg = 10 nm) and (c) TM₃ resonance in the symmetric case. (e) Electromagnetic field distributions of the TM₁, TM₂, and TM₄ resonances in case I (Δg = 10 nm) and TM₃ resonance in symmetric case, respectively.

Fig. 5. (a) Simulated transmission spectra of the S-shaped metasurface in the symmetric and asymmetric cases (Δg = 10 nm, Δd = 10 nm, and θ = 5°) at the normally y-polarized incident wave, respectively. (b) Fano-fitted transmission spectra of TE₁ and TE₃ in case I (Δg = 10 nm). (c)–(e) Q-factors of the TE₁ and TE₃ resonances concerning degrees of asymmetry (Δg, Δd, and θ) for Cases I–III, respectively. Since the TE₁ resonance excited in case II exhibits an extremely weak amplitude, its corresponding variation in the Q-factor is not depicted in (d).

Fig. 6. Scattered powers of the (a) TE₁ and (c) TE₃ resonances in case I (Δg = 10 nm) and (b) TE₂ resonance in the symmetric case. Electromagnetic field distributions of the (d) TE₁ and (f) TE₃ resonances in case I (Δg = 10 nm) and (e) TE₂ resonance in the symmetric case, respectively.

Fig. 7. (a), (b) In case I (Δg = 10 nm), the transmission spectra and corresponding resonance wavelength shifts of three quasi-BICs resonances in different biological environments (refractive index n varies from 1 to 1.1) for TM₁, TM₂, and TM₄. (c), (d) In case I (Δg = 10 nm), the transmission spectra and corresponding resonance wavelength shifts of two quasi-BICs resonances in different biological environments (refractive index n varies from 1 to 1.1) for TE₁ and TE₃.