Fig. 1. Schematic diagram of the evolution process from BIC to chiral q-BIC. (a) The unit cell of a metasurface with a pair of parallel rectangular dielectric pillars that support BIC mode, the lower panel is its transmission spectrum; (b) metasurface with a pair of rectangular dielectric pillars that support a q-BIC mode after introducing the height difference ΔH. The right panel is its transmission spectrum as a function of ΔH. (c) Metasurface with a pair of rectangular dielectric pillars that support a q-BIC mode after breaking the in-plane rotational asymmetry. The right panel is its transmission spectrum as a function of θ. (d) Metasurfaces with a pair of rectangular dielectric pillars that support a chiral q-BIC mode after combining the out-of-plane symmetry-breaking perturbation with the in-plane one. The bottom panel is its transmission spectra under the excitations with opposite circularly polarized lights.

Fig. 2. (a) Schematic diagram of the R-structure that supports a chiral q-BIC mode with periods of P_x = 475 nm and P_y = 350 nm. For the two rectangle pillars, H_R = 110 nm, H_L = 70 nm, W_R = 70 nm, W_L = 110 nm, L = 234 nm. (b) Maximum CD as a function of the rotation angle θ (red line) and the spacing of two pillars W (blue line); (c) transmission spectra depending on the period in the x-direction (P_x) under the excitation of LCP (red lines) and RCP (blue lines), for both the R-structure (lower panel) and L-structure (upper panel). P_x was changed from 445 to 705 nm in steps of 10 nm. (d) ℏΓ_q-BIC as a function of P_x (red dot) and λ_q-BIC as a function of P_x (blue dot). The solid lines are the fitting results. (e) CD values at different P_x, and the inset is the corresponding EF distributions.

Fig. 3. The transmission spectra of the hybrid system with CD values of 0.946 (a), 0.41 (b), and 0 (c), respectively. The red line represents the results under RCP excitation, while the blue line represents that under excitation of LCP. (d) The difference in transmission spectra of the CD values of 0, 0.41, and 0.946, represented by the black, orange, and violet lines, respectively; (e) the transmission spectra of the L-structure under the excitation of RCP and LCP and (f) their difference.

Fig. 4. (a) Transmission spectra of the hybrid system with P_x varying from 445 to 505 nm; (b) simulation results (symbols) of the anticrossing dispersion relationship extracted from (a). Red and blue dots represent the UP and LP, and the corresponding lines are theoretical fitting results. (c) Polariton branch mixing of WSe₂ exciton (green line) and q-BIC (red line) for LP and UP branches; (d) Rabi splitting (ℏΩ_R) as a function of oscillator strength f.

Fig. 5. Rabi oscillation obtained from Eqs. (6) and (7) with different Rabi splitting, ℏΩ_R = 161 meV (a) and ℏΩ_R = 100 meV (b). The other parameters are set to ℏΓ_q-BIC = 8 meV and ℏΓ_ex = 90 meV, Δ = 0. Inset in (a): τ₁ as a function of γ_ex (red line), and τ₂ as a function of ℏΩ_R (blue line). (c) Rabi oscillation with ℏΩ_R = 161 meV, ℏΓ_ex = 45 meV, and ℏΓ_q-BIC = 8 meV. The evolution process of excited states under the excitation of RCP (d) and LCP (e), respectively. Three important interaction times (corresponding to the π/2, π, and 2π) are indicated.

Fig. 6. (a) Schematic of the coupled system with a WSe₂ monolayer on the bottom of the array of silicon pillars; (b) transmission spectra of the hybrid system with P_x varying from 462.5 to 472.5 nm. (c) Simulation results (symbols) of the anticrossing dispersion relationship extracted from (b); red and blue dots represent the UP and LP, and the corresponding lines are theoretical fitting results. (d) Schematic of the coupled system with perovskite pillars array; (e) transmission spectra of the hybrid system with P_x varying from 325 to 365 nm. (f) Simulation results (symbols) of the anticrossing dispersion relationship extracted from (e); red and blue dots represent the UP and LP, and the corresponding lines are theoretical fitting results.