Optical refractive index sensor with Fano resonance based on original MIM waveguide structure

In recent years, optical refractive index sensors have received significant attention in the field of chemical and biological sensing owing to the advantages of small size. In order to achieve the requirements of small structure and high sensitivity of nano refractive index sensing, this paper proposes a Fano resonance-based waveguide structure in which a metal-insulator-metal (MIM) structure using surface plasmons and U-shaped cavities are coupled unilaterally. The transmittance spectrum of the waveguide-coupled resonator with different incident light wavelengths can be obtained. The U-shaped cavity and semi-annular straight waveguide are superimposed on and coupled to each other to form the Fano resonance transmittance spectrum. An extremely narrow impedance is formed in the transmittance spectrum. The proposed structure can obtain superior sensing performance at the nanoscale after the optimisation. The results confirm that the sensing sensitivity of the refractive index of the optimised waveguide structure can achieve up to 825 nm/RIU. The results of experiments show that this structure could accurately identify the five proposed pure edible vegetable oils at room temperature. But experiments have also shown that it could not effectively distinguish between the five proposed mixed edible vegetable oils. This structure provides an effective reference for the design and development of refractive index sensors for application to pure edible vegetable oils.