This paper proposes a novel nano-sized refractive index sensor based on the Fano resonance phenomenon. The main structure consists of two short tubes of the metal-insulation-metal waveguide and an internal Z-ring resonator. The authors used a finite element approach to analyze the nanoscale sensing performance of the system. Simulation results show that asymmetries in the geometry will lead to Fano resonance splitting. This paper explicitly explores whether the structure’s top and bottom asymmetry is a significant factor in the Fano resonance of the internal Z-ring resonator structure. After Fano resonance splitting, the obtained transmission curve was sharper, the bandwidth was significantly reduced, and the system’s figure of merit was significantly improved. This paper further extends the internal Z-ring resonator structure to the sensor field. The resulting refractive index sensor has a sensitivity of 2234 nm/RIU and a figure of merit of 49.65.
