Fig. 1. (a) Schematic diagram illustrating ultrasensitive chiral sensing based on a TD FP-BIC metasurface, which consists of a dielectric metasurface and a multilayer reflector separated by a dielectric spacer. The emergence of TD FP-BIC results from destructive interference between TD resonance and the multilayer reflector-induced perfect mirror image. (b) Comparison among this work and previously reported metasurfaces on Cmax and Cavg for chiral detection.

Fig. 2. Design of the polarization-independent TD metasurface. The transmission spectra of symmetric metasurfaces (a), metasurfaces with single symmetry breaking (b), and asymmetric metasurfaces with dual symmetry breaking (c). Here, TE refers to the electric field polarized along the y direction, and TM refers to the electric field polarized along the x direction. (d) The multipole decomposition of the TD metasurface. More details about multipole decomposition can be found in Appendix A. (e) The Cavg of the TD metasurface under RCP and LCP incidence, respectively. (e) The calculated Cavg of the TD metasurface under RCP and LCP incidence, respectively. The light orange area over the metasurface shown in the inset corresponds to the actual volume used for calculating Cavg. The electric field (f), magnetic field (g), and optical chirality enhancements (h) at z=h/2 under RCP excitation. The electric field (i), magnetic field (j), and optical chirality enhancements (k) at z=h/2 under LCP excitation. The white arrows in (f)–(k) represent the electric displacement fields.

Fig. 3. Design and characterization of the TD FP-BIC metasurface. (a) The transmission spectra of the TD FP-BIC metasurface with respect to the thickness of spacer t. The illustrations depict the schematic diagram of the metasurface. (b) The Cavg of the TD FP-BIC metasurface versus the thickness of spacer t. (c) The multipole decomposition of the TD FP-BIC metasurface. (d) The Cavg of the TD FP-BIC metasurface under RCP excitation. The optical chirality enhancements at z=h/2 for wavelength 1152.44 nm (e), 1152.45 nm (f), and 1152.46 nm (g) under RCP incidence. The white arrows in (e)–(g) represent the magnetic fields.

Fig. 4. Chiral detection based on the TD FP-BIC metasurface. The absorption spectra of chiral molecules alone (a), chiral molecules enhanced by TD metasurface (b), and chiral molecules enhanced by the TD FP-BIC metasurface (c) under RCP and LCP incidence, respectively. The insets show the simulation schematics. (d) CD signals for the three scenarios mentioned above [shown in (a)–(c)]. (e) CD signal enhancements with the assistance of the TD metasurface and TD FP-BIC metasurface.

Fig. 5. Analysis of the symmetric metasurface. (a) The multipole decomposition of the symmetric metasurface under TM polarization. The electric field (b) and magnetic field (c) of the metasurface at z=h/2 under TM polarization. (d) The multipole decomposition of the symmetric metasurface under TE polarization. The electric field (e) and magnetic field (f) of the metasurface at z=h/2 under TE polarization. The white arrows in figures represent the electric displacement fields.

Fig. 6. Analysis of the metasurface with single symmetry breaking. (a) The multipole decomposition of the metasurface with single symmetry breaking under TM polarization. The electric field (b) and magnetic field (c) of the metasurface at z=h/2 under TM polarization. (d) The multipole decomposition of the metasurface with single symmetry breaking under TE polarization. The electric field (e) and magnetic field (f) of the metasurface at z=h/2 under TE polarization. The white arrows in figures represent the electric displacement fields.

Fig. 7. Analysis of the metasurface with dual symmetry breaking. (a) The multipole decomposition of the metasurface with dual symmetry breaking under TM polarization. The electric field (b) and magnetic field (c) of the metasurface at z=h/2 under TM polarization. (d) The multipole decomposition of the metasurface with dual symmetry breaking under TE polarization. The electric field (e) and magnetic field (f) of the metasurface at z=h/2 under TE polarization. The white arrows represent the electric displacement fields.

Fig. 8. Analysis of the reflectance mismatch between the metasurface and multilayer reflector. The reflectance properties of the multilayers with varying numbers of Si and SiO2 pairs: eight pairs used in the main text with reflectance of 100% (a), and three pairs with reflectance of 97% (b). (c) The transmission spectra of the TD FP-BIC metasurface with respect to the thickness of spacer t. The illustrations depict the schematic diagram of the metasurface, which consists of multilayer reflector with lower reflectance. (d) The Cavg of the TD FP-BIC metasurface versus the thickness of spacer t.

Fig. 9. (a) Cavg for the TD FP-BIC metasurface with a −2% deviation in both width and length of the nanostructures. (b) Cavg for the TD FP-BIC metasurface with a +2% deviation in both width and length of the nanostructures. (c) Cavg for the TD FP-BIC metasurface with a −2% variation in the multilayer reflector thickness, spacer thickness, and the height of the nanostructures. (d) Cavg for the TD FP-BIC metasurface with a +2% variation in the multilayer reflector thickness, spacer thickness, and the height of the nanostructures.

Fig. 10. Detailed optical chirality enhancements of the TD FP-BIC metasurface at various heights for different wavelengths. The optical chirality enhancements at different z positions: at z=0 for wavelengths 1152.44 nm (a), 1152.45 nm (b), and 1152.46 nm (c); at z=h/2 for wavelengths 1152.44 nm (d), 1152.45 nm (e), and 1152.46 nm (f); at z=h for wavelengths 1152.44 nm (g), 1152.45 nm (h), and 1152.46 nm (i), respectively. All conditions are under RCP excitation.

Fig. 11. Chiral detection based on the TD FP-BIC metasurface for chiral molecules with n=1.33+10−4i. The absorption spectra of chiral molecules alone (a), chiral molecules enhanced by the TD metasurface (b), and chiral molecules enhanced by the TD FP-BIC metasurface (c) under RCP and LCP incidence, respectively. The insets show the simulation schematics. (d) CD signals for the three scenarios mentioned above [shown in (a)–(c)]. (e) CD signal enhancements with the assistance of the TD metasurface and TD FP-BIC metasurface.

Fig. 12. Concept of the gradient metasurface for broad spectral range detection. (a) Schematic representation of a series of gradient metasurfaces in which the in-plane dimensions of the resonators increase gradually from left to right. The structural parameters, length a and length b, are identical to those in Fig. 2, and the factor s denotes the applied scaling factor. (b) Numerical results for Cavg of the TD FP-BIC metasurfaces with scaling factors ranging from 0.98 to 1.02. The peak resonance of Cavg shifts from 1130 nm to 1170 nm, while maintaining high enhancement values, showcasing the potential for border spectral range detection.