Herein, a refractive-index sensor is theoretically proposed, which comprises metal-insulator-metal waveguide and single rectangle cavity without a long side (RCWALS). The propagation characteristics were analyzed by finite element method. Compared with the contrastive structure, the designed single RCWALS structure has better transmission spectra and stronger resonance and supports Fano resonance. The effects of the structural parameters on sensing characteristics were investigated. Mode 1 of

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... ngle RCWALS structure has better performance parameter, which is the best sensitivity of 1840 nm/RIU with a figure of merit of 51.11. Additionally, three derived structures are presented, which are composed of MIM waveguide and two RCWALS in different positions. They have lower sensitivity but better figure of merit, and provide more detection positions. The designed structures, which are greatly simple, can potentially apply to nanophotonics. Index Terms: Metal-insulator-metal, Fano resonance, refractive-index nanosensor, finite element method.