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We propose an ultrasmall plasmonic cavity 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 with environmental material, the proposed plasmonic cavity can be used as one of best candidates for sensitive refractive index sensor. 

plasmonics;cavity;nanophotonics