Fig. 1. Structure diagram of MIM waveguide coupled with special square cavity

Fig. 2. Transmission spectra of SPPs waveguide structure. (a) Both baffle and square cavity exist; (b) single baffle and single square cavity

Fig. 3. Magnetic field distribution of SPPs waveguide at different incident wavelengths. (a) Magnetic field distribution at valley $\lambda$=1344 nm; (b) magnetic field distribution at FR1 $\lambda$=1362 nm; (c) magnetic field distribution at FR2 $\lambda$=2222 nm

Fig. 4. Transmission spectra and resonance wavelengths with different radii R. (a) Transmission spectra for different radii R; (b) linear fitting relationship between Fano resonance wavelengths of FR1、FR2 and R value

Fig. 5. Transmission spectra and resonance wavelengths with different refractive indices of medium. (a) Transmission spectra for different refractive indices of medium; (b) linear fitting relationship between Fano resonance wavelengths and refractive index of medium

Fig. 6. f_FOM of the SPPs waveguide structure under different wavelengths

Fig. 7. Transmission spectra and resonance wavelengths with different mass concentrations of glucose in glucose solution. (a) Transmission spectra for different mass concentrations of glucose in glucose solution (R=180 nm,d=20 nm, g=a=10 nm); (b) linear fitting relationship between Fano resonance wavelengths and mass concentrations of glucose in glucose solution

Table 1. Comparison of refractive index sensitivity and f_FOM with other references