Standing-wave resonances in plasmonic nanoumbrella cavities for color generation and colorimetric refractive index sensor

[Display omitted] •Scallop effect in Bosch process to realize the nanoscale fabrication of umbrella-shape plasmonic metasurfaces.•We experimentally demonstrate that the nanoumbrella-cavity resonator exhibits standing-wave resonances in visible region.•Energy confinement by multiple reflections in the umbrella-cavity, and hence resulting in narrowband resonance.•Vivid and subtle colors achieved through overlaying spectral colors defined by standing-wave resonant wavelengths.•Narrow-bandwidth multicolor metasurfaces enable the realization of a sensitivity colorimetric refractive index sensor. We theoretically investigate the hybridization of the elemental surface plasmons in umbrella-shape plasmonic nanostructures and experimentally demonstrate the implementation of plasmonic multicolor metasurfaces as well as their application in colorimetric sensing. The three-dimension metallic umbrella arrays consist of a periodic canopy-capped-nanopillars with metal-coated sidewall and a backplane metal-film to form vertical nanocavity of canopy and film. Plasmonic coupling and energy confinement in nanocavity induce a noticeably resonance narrowing of multispectral reflection. The metasurfaced nanostructures appeared in vibrant and tunable colors with broad gamut derived from color blending mechanism due to multiple, narrow-band resonances. Vivid colors varied from red, yellow, green, blue to violet are easily achieved. It is also shown that such plasmonic metasurfaces can work as the feasible and real-time colorimetric refractive index sensor by measuring the distinct color variation to glucose concentration changes. Our sensor scheme shows its spectral sensitivity in the periodic umbrella array with respect to the refractive index change to be 242.5nm/RIU with a figure of merit of 7.3. Furthermore, a refractive index resolution of colorimetric sensing up to 0.025 RIU has been accomplished.