Narrow-band notch filter and refractive index sensor based on rectangular-semi-annular cavity coupled with MIM waveguide structure

In this work, a symmetric structure based on a metal-insulator-metal (MIM) waveguide is proposed for narrow-band notch filters and refractive index sensors. The structure comprises a symmetrically intersecting rectangular-semi-annular cavity and a long straight waveguide. The transmission spectrum and magnetic field distribution of the notch filter are studied and analyzed by the finite element method (FEM) with scattering boundary conditions (SBC). Our analysis reveals that the band-stop filter exhibits a minimum transmittance of 0.35%, a bandwidth of 34 nm, and a quality factor (Q) of 35.16. These characteristics include low stop-band transmittance, a narrow bandwidth, and a high Q value. Furthermore, our structure allows for independent adjustment of the narrow band filtering range by varying geometric parameters such as H, d, R1, and d. Additionly, we conduct theoretical analysis to investigate the sensing properties of the structure. The refractive index sensitivity of the structure reaches a high value of 1222 nm RIU-1, accompanied by a figure of merit(FOM*) of up to 175.9. These results demonstrate the structure's excellent filtering properties and high-sensitivity sensing characteristics.Consequently, it holds significant potential for application in high-density integrated circuits and nano-optics.