Plasmonic Perfect Absorber Based on Graphene and Its Sensing Application

A plasmonic perfect absorber is proposed using a silver substrate in the visible near-infrared wavelength range. The unit cell of the periodic structure is designed and evaluated in this paper. The Ag layer is etched so that four trapezoidal trenches filled with Si3N4 couple plasmonic modes on the metal-insulator interfaces. Various optical modes are formed, from which, a surface and a bulk mode are thoroughly studied. The surface mode can be coupled to the graphene sheet surface through a thin dielectric layer. Moreover, the effect of the variation of the structure's geometrical parameters on these modes was discussed. The bulk mode changes by the volume of the trenches and the effective refractive index, whereas the surface mode shifts with the period of the unit cell, the trenches depth, and other parameters that change the coupling criteria. The variation of the surface mode of the proposed structure can be used in sensors and photodetectors. As an application instance, an optimized structure is proposed at the end of the paper for sensing applications, showing high sensitivity (560 to 600 nm/RIU) and a figure of merit (70 similar to 120 RIU-1) in a wide refractive index range.