Multiple Fano resonance modes can be excited by the introduction of the asymmetric notch in a single closed ring cavity, but the change of any structural parameter will cause synchronous changes in the resonance mode, which is not conducive to the flexibility of practical sensing applications. We aim to design a metal?insulator?metal (MIM) waveguide structure, which can generate uncorrelated double Fano resonance in the infrared region to prepare a label-free biosensor with high sensing performance and high flexibility.

To this end, we introduce a single metal baffle and two split-ring resonator cavities with different radii on the surface of the MIM waveguide, as shown in Fig. 1(a). Since the penetration depth of the electromagnetic field in the waveguide is much less than its wavelength, the original three-dimensional structure can be replaced by a two-dimensional structure. The metal baffle can stimulate the broadband mode, and two split-ring resonator cavities with different radii can stimulate two narrow-band modes. The broadband mode and narrowband mode are coupled to produce a double Fano resonance mode. By adopting the finite element method, the transmission characteristics of light waves and the distribution of electromagnetic fields on the structure surface are calculated theoretically. To discuss the modulation effect of structural parameters on the Fano resonance mode, we calculate and compare the transmission spectra at different radii and opening angles. In addition to studying the movement of the resonant modes and sensing sensitivity of the structure, we investigate whether the two Fano resonance modes can be modulated independently.

The calculation results based on the finite element method show that the proposed structure can excite typical double Fano resonance, as shown in Fig. 2. The corresponding wavelengths of the two transmission peaks are 1481 nm and 2484 nm respectively. Additionally, the electromagnetic field distribution on the waveguide surface is simulated. By theoretical calculation, the sensitivity of a resonator with a radius of 200 nm is 1453 nm/RIU, and that of a resonator with a radius of 300 nm is 3793 nm/RIU. We also prove that the Fano resonance mode can be modulated effectively by changing the radius and opening angle of the ring cavity, including the resonance wavelength position, peak value of the transmission peak, sensing sensitivity, and other properties. The increase in the radius of the split-ring resonator cavity will cause the red shift of the resonance wavelength, as shown in Figs. 5(a) and (c). The opening angle increase will cause the blue shift of the resonance wavelength, as shown in Figs. 6(a) and (b). In addition, these figures confirm that both Fano resonance modes can be modulated independently, which provides unique flexibility in label-free biosensing.

We successfully design a MIM waveguide structure capable of exciting double Fano resonance, achieving ultra-high sensing sensitivity and unique flexibility in the near-infrared band. We introduce a single metal baffle and two split-ring resonator cavities with different radii into the MIM waveguide, which are coupled to produce double Fano resonance. The numerical results based on the finite element method show that our structure has very high sensitivity and a good quality factor. By changing the radius of the ring cavity or the size of the opening angle, the resonance mode position, peak value of the transmission peak, sensing sensitivity, and other attributes can be flexibly modulated. Additionally, since the two resonance modes originate from different resonators, they can be independently regulated, which has significant advantages in practical sensing applications. Our structure provides new ideas for highly sensitive and flexible label-free biosensing.