A high sensitivity refractive index sensor have been proposed in this paper, which is composed by an M-shaped resonant cavity coupled with a baffle contained Metal-Dielectric-Metal (MDM) waveguide. The influence of structural parameter of M-shaped cavity and filled medium characteristics inside it on the transmission spectrum of MDM waveguide have been analyzed numerically through the finite element method. Simulation results show that four asymmetric transmission peaks with Fano line-shape are generated by the coupling interference between the four quasi Fabry-Perot (FP) resonant modes of the first to fourth orders inside M-shaped cavity and the reflected wave in MDM waveguide, moreover the four peak wavelengths redshift almost linearly with increasing cavity length or the refractive index of the filled medium, which can be used to design the refractive index sensor. The calculated sensitivities associated with the quadruple Fano transmission peaks are found to be proportional to the cavity length and the reciprocal of resonant order. As a consequence, a sensitive up to 4 900 nm/RIU has been realized by an M-shaped resonator in dimensions of 400 nm×400 nm. The results in the article provide an effective guidance for the design of high sensitivity sensors.

Based on the well-known analogy of propagation modes between metal-insulator-metal (MIM) waveguides and microwave transmission lines, an improved transmission line model (ITLM) have been proposed to describe reflection coefficients of a MIM waveguide stub. The Fresnel formula have been employed to predict the phase shift resulting from the reflection of surface plasmon polaritons (SPP) from the stub's end, which allows the ITLM to describe the influence of the stub-shaped resonators arranged in the side-coupled configuration on the transmission spectrum of main waveguide more precisely. In this paper, the improved transmission line models of MIM waveguide side coupled to single stub and double-side symmetric stubs have been proposed, and the predicted transmission spectrum by the ITLM is well agreed with simulation results of the finite element method. Specifically, transmission minimum predicted by ITLM is more close to that calculated by numerical methods than the classical transmission line model. Moreover, it is verified that ITLM possess much higher efficiency in calculating the transmission spectrum of the MIM waveguide connected with stub-shaped resonators than the finite element method employed in this work.