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Graphene is an atomically thin carbon layer with a two-dimensional hexagonal lattice structure and has rich optoelectronic properties well suited to a wide range of applications. Graphene is considered to be a promising material for photodetectors because it exhibits excellent properties such as broadband absorption covering at least the ultraviolet to terahertz frequencies. However, the low optical absorption of graphene, at ca. 2.3%, still remains an important problem. Plasmonic metamaterial structures are good candidates to address this challenge. Metal-insulator-metal-based plasmonic metamaterial absorbers (MIM-PMAs) are highly suitable for the introduction and application of graphene. MIM-PMAs have a multilayer structure that includes plasmonic micropatches, an insulator, and a metal reflector layer. MIM-PMAs exhibit wavelength-selective absorption according to the micropatch size. Our previous research has demonstrated that the optical absorption of graphene is enhanced only by the main plasmonic resonance mode, and the plasmonic resonance modes in MIM-MPAs are strongly influenced by the insulator material. Therefore, the insulator layer plays an important role in graphene-coated MIM-PMAs. In this study, we have investigated the effect of the insulator layer in graphene-covered MIM-PMAs. The graphene was fabricated by chemical vapor deposition and transferred onto MIM-PMAs with different insulator thicknesses. Reflectance measurements demonstrated that varying the insulator thickness had a significant effect on the absorbance of graphene and resulted in modulation of the absorption wavelength. These results indicate that the plasmonic resonance localized at graphene near the plasmonic micropatches is modulated by the waveguide mode in the insulator layer. We believe that the present study will lead to significant improvements in graphene-based infrared detectors.
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