Fig. 1. (a) Schematic for the dimer metasurface. The dimers are arranged in a square lattice with a period of P. The parameters of the dimer unit cell are shown in the lower left corner, where l_x, l_y, and l_z control the size of the dimer, and d₁ and d₂ determine the symmetry properties of the unit cell. (b) Illustration for an unperturbed unit cell with d₁ = d₂ = d₀ = (P − 2 l_x)/2; (c) unit cell with RS breaking characterized by a nonzero Δd = d₁ − d₂ and d_avg = (d₁ + d₂)/2 = d₀; (d) unit cell with TS breaking characterized by a nonzero Δd_avg = (d₁ + d₂)/2 − d₀ and Δd = 0; (e) unit cell with both TS and RS breaking with both nonzero Δd and Δd_avg; the dashed frames in (b)–(e) indicate the location of the unperturbed dimer unit cell.

Fig. 2. Surface charge density and vectorial electric field distribution of (a) ED_o − BIC and (b) ED_i − BIC in an unperturbed dimer unit cell. (c) Transmission spectra of a TS-perturbed dimer metasurface as a function of Δd_avg; the transmission dip corresponds to ED_o − QBIC, and Δd = 0. (d) Transmission spectra of an RS-perturbed dimer metasurface as a function of Δd; the transmission dip corresponds to ED_i − QBIC, and Δd_avg = 0. Circles indicate the nonradiative BICs. The Q factors of ED_o − QBIC and ED_i − QBIC against (e) TS-only and (f) RS-only perturbation.

Fig. 3. (a) Normalized dipolar components in ED_o − QBIC and ED_i − QBIC resonances with different Δd_avg and Δd perturbations; (b) surface charge distribution and magnetic field of ED_o–QBIC in multipole unit cells of a dimer metasurface with (top) Δd_avg = 0 and (bottom) Δd_avg = 60 nm.

Fig. 4. (a) Transmission spectrum of a dimer lattice with different TS and RS perturbation strengths characterized by Δd and Δd_avg; Q factors of (b) ED_o − QBIC and (c) ED_i − QBIC as a function of Δd and Δd_avg.

Fig. 5. (a) Absorption spectrum: ED_o − BIC against TS-only perturbation with Δd = 0; the dashed line indicates that the absorption reaches 50%, corresponding to Δd_avg = 72.1 nm. (b) Absorption spectrum: ED_i − BIC against RS-only perturbation, Δd_avg = 0; the dashed line indicates that the absorption reaches 50%, corresponding to Δd = 33 nm. Circles indicate the formation of BICs. (c) corresponds to the dashed line in (a), and (d) corresponds to the dashed line in (b). Radiative Q-factor Q_R and nonradiative Q-factor Q_N of (e) ED_o − BIC and (f) ED_i − BIC. The arrows represent the critical coupling points, corresponding to the perturbations Δd_avg = 73 nm and Δd = 35 nm, respectively.

Fig. 6. Resonant wavelengths of ED_o − BIC and ED_i − BIC as a function of (a) l_y and (b) l_z.

Fig. 7. Absorption spectra of the asymmetric dimer metasurface with (a) l_x = 100 nm; (b) l_x = 150 nm; and (c) l_x = 200 nm, with the resonance crossing triggered by tuning l_y. The white dashed line indicates near-unity absorption occurs, and the lower panels show the corresponding absorption spectra.