Fig. 1. Schematic of double layer metal film grating with composite periodic structure. (a) Three-dimensional diagram of the designed structure; (b) cross-section of the designed structure

Fig. 2. Characteristic spectra of several structures. (a) Calculated spectra of reflection、absorption and transmission for the designed structure; (b) comparison of reflection spectra for non-composite and composite periodic structures; (c) comparison of absorption spectra for non-composite and composite periodic structures

Fig. 3. Sensing performances for the designed structure. (a) Simulated reflection spectra with various refractive indexes; (b) relations between resonance wavelengths at reflected dips and different refractive indexes; (c) figure of merit (FOM) of resonant dips; (d) intensity responses of new dip and the third dip with various refractive indexes

Fig. 4. Magnetic field distributions at different resonance wavelengths for (a) the first reflection dip, (b) the second reflection dip, (c) the third reflection dip and (d) the forth reflection dip. Here, (a1) depicts the direction and intensity of surface current density and (a2) depicts surface charge distribution at the slit of the nearest substrate for the first reflection dip

Fig. 5. Electric field distributions at different resonance wavelengths. (a) The first reflection dip; (b) the second reflection dip; (c) the third reflection dip; (d) the forth reflection dip

Fig. 6. Effects of different parameters on the reflection spectra for designed structure. (a) Slit width (m); (b) dielectric interlayer thickness (T₂); (c) thicknesses (T₁ and T₃) of two metal films for top and down, respectively; (d) grating ridge width (W₂); (e) grating period (P)