We propose a high-quality multispectral plasmonic sensor by coating a dielectric grating on the semiconductor-metal-semiconductor stacks. Ultra-narrow plasmonic resonances with the minimum bandwidth of 8 nm are insensitive to the polarization of incident light. The ultra-narrow multi-band phenomenon can be ascribed to the synergy of guided mode resonances, cavity modes and surface plasmon polaritons. The sensor displays superior refractive index sensitivity (404.295 nm/RIU) and figure of merit (50.3) and can also be used to detect the protein layer in sub-nanometer thickness. These offer new perspectives for achieving ultra-compact efficient sensors and filters.

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