Fig. 1. Schematic of the inverse design method for a chiral BIC metasurface. (a) Flowchart of meta-atom structure design, including diagrams of the BIC structure, the quasi-BIC structure, and topologically optimized chiral quasi-BIC structure. (b) Schematic diagram of the topological optimization process. (c) Functionality diagram of the circularly polarized chiral BIC metasurface.

Fig. 2. Simulated reflection spectrum during the topological optimization process. (a) Reflectance spectrum of the initial structure for X and Y polarizations (Px=760nm, Py=380nm, L1=300nm, L2=280nm, W1=160nm, W2=150nm, λ0=1307nm). (b) Iteration process diagram of FoM parameters; inset: iterative evolution of the free-form structure. (c)–(g) Reflectance spectra of the structures after the 100th, 200th, 300th, and 400th iterations.

Fig. 3. Simulated multipole contributions and near-field distributions during the topological optimization process. (a)–(e) Multipole decomposition spectra of the chiral BIC. (f)–(j) Electric field distribution in the xz plane, with black arrows representing the induced current. (k)–(o) Magnetic field distribution in the xy plane, with white arrows representing the magnetic vector.

Fig. 4. Experiment results of the fabricated BIC metasurfaces. (a) Schematic of the experimental setup for spectral measurement of transmitted light through chiral BIC metasurfaces. (b), (c) Measured transmission spectra of fabricated quasi-BIC metasurface (b) and topologically optimized chiral BIC metasurface (c); inset: SEM images of the fabricated metasurfaces. (d) Simulated and measured CD of the chiral BIC metasurfaces.

Fig. 5. Simulation and experimental spectral curves of the chiral quasi-BIC metasurface devices. (a) Metasurface simulation with amorphous silicon on glass with the following parameters: meta-atom period Px=720nm, Py=360nm, λ0=1225nm, Q=441; inset: topologically optimized meta-atom structures. (b) Experimental spectral curves of (a). Inset: partial scanning electron microscope (SEM) images of the respective metasurfaces. (c) CD results for the simulations and experiments; the values are as follows: CDsim=−0.63, CDexp=−0.6.

Fig. 6. Optimization for specific elliptic polarizations. (a) The target polarization states on the Poincaré sphere. (b)–(e) The unit structures and performance of the optimized quasi-BIC metasurfaces with polarization states corresponding to positions 1–4. Inset: unit cell structures of the respective metasurfaces. The input polarization state α(ψ,χ) values are as follows: α(0,−π/6), α(0,−π/12), α(0,π/12), and α(0,π/6).

Fig. 7. The simulated multipole contributions and near-field distributions during the topology optimization process at the 0th, 100th, 200th, 300th, and 400th iteration steps. (a)–(e) Multipole decomposition spectra of the chiral BIC. (f)–(j) Electric field distribution in the xz plane, with black arrows representing the induced current. (k)–(o) Magnetic field distribution in the xy plane, with white arrows representing the magnetic vector.

Fig. 8. (a) Simulated transmission curves for each polarization component. (b) Simulated transmission curves for left- and right-handed circular polarizations (Tl=0.11). (c) Experimental transmission curves. (d) Simulated reflection curves for each polarization component. (e) Simulated reflection curves for left- and right-handed circular polarizations (Rl=0.89). (f) Comparison of simulated and experimental CD values.

Fig. 9. (a)–(d) Meta-atom structural variations due to erosion and dilation. (a) The structure with a 10 nm overall erosion; (b) the original structure; (c) the structure with a 10 nm overall dilation; (d) the structure with a 10 nm partial dilation; (e)–(h) the reflection spectral curves of meta-atom structures. The resonance wavelengths are 1261 nm, 1307 nm, 1369 nm, 1333 nm, respectively, and the FWHM of (h) is 7 nm.

Fig. 10. Effect of different material losses on the transmission performance of super surface.