Surface plasmon modes possesses an intriguing feature of confining light beyond diffraction limit which makes it very attractive for sensing applications. Here, we theoretically investigated an ultra compact surface Plasmon (SP) sensor using Metal-Insulator-Metal (MIM) waveguide geometry. MIM waveguide is coupled to a pair of stub resonators and both the stub resonators are loaded with a metallic nano slit of silver. The stubs and MIM waveguide are filled with liquid/ gaseous material which is to be sensed. The Fano resonance which is very sensitive to any change in refractive index of the material, is excited in the structure by breaking marginal symmetry. The Structure is numerically simulated by Finite Difference Time Domain (FDTD) method and the result shows that resonance wavelength has a linear relation with refractive index of the material under sensing. In the optimum design of proposed sensor, the maximum sensitivity is obtained as high as S= 1060 nm/RIU with large value of Figure of merit (FOM=176.7) and an ultra narrow linewidth ∆ = . Thus, the device is well suited for designing on-chip optical sensors.

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