Fig. 1. Nanograting coupled multilayer stack structure for improving sensing figure of merit. (a) Three-dimensional schematic for biomolecule detection; (b) Cross-sectional view of the structure and corresponding material composition. The red arrows represent the propagation path of the TM polarized light, and the red curves represent the distribution characteristics of the electric field of SPP and TPP in the structure

Fig. 2. The reflection spectra of the simplified structure. When the incident angle is equal to the first-order diffraction angle of the nanograting with a period of 536 nm and the thicknesses of the gold film on the top surface of the structure are (a) 400 nm and (b) 40 nm, respectively, the effect of the center wavelength of the photonic crystal on the reflection spectra of the gold film-photonic crystal multilayer structure; (c) The relationship between the wavevector and the energy of the incident light in the structure, and the dispersion curves excited separately by the SPP mode (yellow short dashed curve) and the low-order TPP mode (cyan short dashed line) ; (d) The reflection spectra of the structure that only excites TPP mode (yellow curve) and SPP mode (blue curve), and the reflection spectra corresponding to the strong coupling between TPP and SPP (red curve)

Fig. 3. Reflection spectra and electric field distributions at the resonance positions of the grating-coupled multilayer stack structure. (a) Reflection spectra of three typical structures, in which there are TPP (yellow curve) and SPP (blue curve) modes and TPP-SPP hybrid mode (red curve) generated; The spatial electric field intensity distributions of (b) SPP mode, (c) TPP mode; (d) High-frequency and (e) low-frequency TPP-SPP hybridization modes. The curve in the right inset is the variation of electric field intensity at the dotted line position in the electric field intensity distribution diagrams

Fig. 4. Quantitative evaluation of the sensing performance. Reflection spectra of structures with (a) TPP-SPP hybrid mode and (b) SPP mode at the ambient with different refractive indexes; (c) The amount of red-shift of the resonance wavelengths caused by the change of the external refractive index; (d) Sensing figures of merit of TPP-SPP hybrid mode (red mark) and SPP mode (blue mark) under the ambient surroundings with different refractive indexes

Fig. 5. Reflection spectra of the structures corresponding to the excited high-frequency TPP-SPP hybrid mode structures at the wavelengths of (a) 631 nm and (b) 844 nm. The reflection spectra of the SPP mode are used as a reference; (c) Bulk refractive index sensitivity and (d) sensing figure of merit of TPP-SPP hybrid mode and SPP mode at different structural periods