Fig. 1. Schematic diagram of the sensor structure. (a) Heterocentric ring array structure; (b) top view of the cell structure

Fig. 2. Reflection characteristics under different centroid distances d. (a) Reflectance spectra; (b) R_dip of dip 1 and dip 2 in the range of d=10‒40 nm; (c) FWHM of dip 1 and dip 2 in the range of d=10‒40 nm; (d) R_dip and FWHM of dip 2 in the range of d= 0‒10 nm; (e)‒(h) electric fields and electric field vector distributions on the lower surface of the ring

Fig. 3. Spectral fitting curves with different centroid distances d. (a) d=10 nm; (b) d=20 nm; (c) d=30 nm; (d) d=40 nm

Fig. 4. Reflective characteristics of the structure with different outer ring radii R. (a) Reflectance spectra; (b) R_dip and FWHM of dip 2; (c)‒(e) electric fields and electric field vector distributions of the lower surface of the ring

Fig. 5. Reflective characteristics of the structure with different inner cavity radii r. (a) Reflectance spectra; (b) R_dip and FWHM of dip 2;

Fig. 6. Reflective characteristics of the structure with different ring thicknesses h. (a) Reflectance spectra; (b) R_dip and FWHM of dip 2; (c)‒(e) electric fields and electric field vector distributions of the lower surface of the ring

Fig. 7. Reflective characteristics of the structure with different array periods P and different silica layer thicknesses t. (a) Array period P; (b) silica layer thickness t

Fig. 8. Sensing performance analysis. (a) Effects of the external refractive index n on the reflectance spectra; (b) variation of resonance wavelength with the external refractive index n; (c) variation of the light intensity of the reflected valley with the external refractive index n