###### Abstract  
In this article, a novel refractive index sensor composed of a metal-insulator-metal (MIM) waveguide with two rectangular stubs coupled with an elliptical ring resonator is proposed, the geometric parameters of which are controlled at a few hundreds of nanometer size. The transmission feature of the structure was studied by the finite element method based on electronic design automation (EDA) software COMSOL Multiphysics 5.4 (Stockholm, Sweden). The rectangular stub resonator can be thought of as a Fabry-Perot (FP) cavity, which can facilitate the Fano resonance. The simulation results reveal that the structure has a symmetric Lorentzian resonance, as well as an ultrasharp and asymmetrical Fano resonance. By adjusting the geometrical parameters, the sensitivity and figure of merit (FOM) of the structure can be optimized flexibly. After adjustments and optimization, the maximum sensitivity can reach up to 1550 nm/RIU (nanometer/Refractive Index Unit) and its FOM is 43.05. This structure presented in this article also has a promising application in highly integrated medical optical sensors to detect the concentration of hemoglobin and monitor body health. surface plasmon polaritons; refractive index sensor; sensitivity; Fano resonance; metal-insulator-metal waveguide +
Footnote †: journal: Applied  
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a large number of devices related to MIM waveguides to realize a variety of instruments that have different functions, such as filters [6; 7; 8; 9; 10; 11], splitters [12; 13], sensors [14; 15; 16; 17; 18; 19; 20; 21], and they greatly promote the development of novel instruments. Compared with Lorentzian resonance, Fano resonance has been applied widely in sensing instruments because of its ultrasharp and asymmetrical transmission spectrum, which is sensitive to the changes of the structure's parameters and the surrounding environment [22; 23; 24; 25; 26]. There are two vital parameters that evaluate the sensing performance of the system: the sensitivity S and the figure of merit (FOM). Domestic and foreign scholars have recently proposed and studied a variety of sensing structures that make full use of the sensitivity of the Fano resonance to realize different applications. [PERSON] et al. [27] designed an MIM waveguide coupled with a pair of elliptical ring resonators; the sensitivity can reach up to 1100 nm/RIU and its FOM is 224. A compact refractive index nanosensor composed of an MIM waveguide coupled with a circular split-ring resonant cavity was proposed by [PERSON] et al. [28], whose sensitivity is up to 1500 nm/RIU and the FOM can reach 65.2. [PERSON] [29] proposed a structure composed of an MIM waveguide and rectangular side-coupled cavity, whose sensitivity is 1280 nm/RIU. [PERSON] also found that by means of increasing the number of rectangular resonance cavities, researchers can get more Fano resonance peaks. The FOM of the M-type resonant cavity structure designed by [PERSON] [30] et al. is up to \(1.56\times 10^{5}\), but its sensitivity is only 780 nm/RIU. In this article, a structure composed of an MIM, two symmetric rectangular stubs and the elliptical ring resonator, is investigated. The rectangular stub resonator can be thought as a Fabry-Perot (FP) cavity, which can facilitate the Fano resonance. The sensitivity and FOM are studied by means of trying to alter different geometrical parameters of the system. According to numerous simulations, the proposed system can serve as a highly efficient refractive index sensor with a sensitivity of 1550 nm/RIU and FOM of 43.05. [PERSON] et al. [31] proposed that the measured values of the refractive index have a linear relationship with the concentration of hemoglobin at all wavelengths, and showed a linear relationship between them. Herein, the Fano resonance relies on its advantage that it is sensitive to the changes of refractive indices, making it become a promising platform for designing highly integrated medical optical sensors to detect the concentration of hemoglobin and monitor body health.