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We propose an ultrasmall plasmonic cavity for refractive index sensor based on channel-waveguides of different widths. Plasmonic mode-gap due to cutoff frequency enables strong optical confinement in a subwavelength volume and also suppression of radiation loss. Because of strong field overlap of the SPP mode with environmental material and small cavity size, the proposed plasmonic cavity is an ideal building block for sensitive refractive index sensor. Large sensitivity (S = 1130 nm/RIU) and the figure of merit (FOM = 270) are achieved in the small cavity size (600 nm × 800 nm × 1400 nm). This open-type channel-waveguide plasmonic cavity can open up broad range of applications including biochemical sensing and strong light-matter interactions.

plasmonics;cavity;nanophotonics