###### Abstract  
A plasmonic refractive index nanosensor structure consisting of a metal-insulator-metal (MIM) waveguide with two symmetrical rectangle baffles coupled with a connected-concentric-double rings resonator (CCDRR) is presented. In this study, its transmission characteristics were investigated using the finite element method (FEM). The consequences, studied via simulation, revealed that the transmission spectrum of the system presents a sharp asymmetric Fano profile due to the destructive interference between the wide-band mode of two rectangle baffles on the bus waveguide and the narrow-band mode of the CCDRR. The effects of the geometric parameters of the structure on the transmission characteristics were investigated comprehensively. A sensitivity of 2260 nm/RIU and figure of merit (FOM) of 56.5 were the best levels of performance that the designed structure could achieve. In addition, the system could act as a sensor for use for temperature sensing, with a sensitivity that could reach 1.48 nm/\({}^{\circ}\)C. The designed structure advances with technology with new detection positions and has good application prospects in other high-sensitivity nanosensor fields, for example, acting as a biosensor to detect the hemoglobin level in the blood.  
plasmonic refractive-index nanosensor; Fano resonance; metal-insulator-metal waveguide; temperature sensor +
Footnote †: journal:Nowadays, many waveguide coupling structures based on SPPs have been designed to fabricate various photonic devices, including metal\(-\)insulator\(-\)metal (MIM) waveguides [16], insulator\(-\)metal\(-\)insulator waveguides [17], channel waveguides [18; 19], and nanoparticle chain waveguides [20]. Among them, MIM waveguides are widely considered and reported by scholars and the media at home and abroad because of their sub-wavelength size, simple structure, easy integration, and high reliability [16; 21]. [PERSON] et al. [22] designed a coupling structure with a double-gap ring cavity and MIM waveguides with two triangular baffles with a sensitivity of 1500 nm/RIU and figure of merit (FOM) of 65.2. [PERSON] et al. [23] proposed and studied a plasmonic structure that includes a ring nanocavity, two bus waveguides, and a rectangular nanocavity, with a sensitivity of 1125 nm/RIU. [PERSON] et al. [24] devised a plasmonic sensor coupled with an elliptical ring cavity and a MIM waveguide with two rectangle baffles; its sensitivity is 1550 nm/RIU and FOM is 43.05. As shown in Table 1, although the difference of FOM is not obvious, the sensitivity of the designed structure is obviously better than that of other structures. In addition, various photonic devices based on MIM waveguide structure design, such as optical splitters [25; 26], filters [27; 28], and Bragg reflectors [29; 30], have achieved remarkable results.  
Herein, a plasmonic structure consisting of a MIM waveguide with two symmetrical rectangle baffles coupled with a connected concentric double rings resonator (CCDRR) is presented and investigated. The transmission characteristics and the normalized magnetic field distribution were calculated, introducing the finite element method (FEM). In addition to the influence of the refractive-index of the dielectric on the transmission characteristics of Fano resonance, the influence of the geometric parameters of the structure was also studied. These parameters include the external radii of the outer ring and inner ring of the CCDRR, the separation between the two symmetrical rectangular baffles, the heights of the two rectangular baffles, and the coupling gap between the CCDRR and the bus waveguide. Additionally, applications of the designed structure in refractive-index sensing and temperature sensing were studied in detail. The designed structure provides new detection positions, which may be helpful for meeting special requirements for detection wavelengths.