In order to achieve a highly sensitive refractive index sensor in a nanoscale spatial dimension, a square composite resonant cavity containing a stub resonator has been proposed as a resonant element adjacent to Metal-Dielectric-Metal (MDM) Surface Plasmon Polaritons (SPPs) waveguide in this paper.

The paper employs the finite element method to comparatively analyze the characteristics of the transmission peak spectrum and the mode field distribution at the peak position of the square resonant cavity coupled MDM waveguide structure before and after the introduction of the stub cavity. The analysis results reveal that the degeneracy separation phenomenon of the first and second order quasi resonant modes in the composite cavity can be observed clearly when the stub resonator is added in the square-shaped cavity. Consequently, four transmission peaks with sharp edges have emerged in the transmission spectra, which are generated by the quadruple Fano resonance. By changing the structural parameters such as the side length of the square cavity and stub length, as well as the refractive index of the medium filled in the cavity, the dependence of the transmission spectra on the structural and material parameters of the resonant cavity was calculated in detail.

By analyzing the quantitative impact of structural parameters on the peak wavelength, the resonance conditions responsible for the transmission peaks have been derived and the increments of the effective cavity length contributed by the stub resonator have been analyzed in turn for the four resonant modes. By calculating the transmission spectra associated with the varied medium filled in the composite cavity, the peak wavelength shows an approximate linear relationship with the refractive indexes of the filled medium. The sensing sensitivity of the quadruple transmission peaks has been obtained based on the linear fitting results.

The calculation results in the article indicate that the sensitivity of refractive index sensing is directly proportional to the effective cavity length of the composite cavity, and inversely proportional to the resonance order. When selecting a square composite cavity structure with a side length of 360 nm and a stub length of 210 nm, the sensing sensitivity of the transmission peak produced by the first-order Fano resonance can reach 2 000 nm/RIU. The research conclusion of this paper can provide a valuable reference for the design of nanoscale high sensitivity refractive index sensors.