All-metal plasmonic metasurface at NIR wavelengths for optical absorption manipulation and refractive index sensing

All-metal near-infrared (NIR) plasmonic perfect absorbers made of subwavelength rings and ringdiscs on a flat metal layer are proposed and numerically investigated using FDTD solver. A simple design concept is applied to achieve multi-resonant absorption. Ring inner diameter and outer diameter are set equal to p-200 nm and p-100 nm, respectively, where, p = period of the structure, and a disc of appropriate size is co-axially introduced at the centre of the gold ring to controllably achieve 100% absorption. Passive controlling of the disc size changes single absorption mode to dual absorption modes. Further, multiple resonant absorptions at tunable NIR wavelengths are achieved by embedding the structures into a dielectric medium of high dielectric constant. The proposed structure is seen to be highly sensitive to the surrounding medium that can measure refracting index with high sensitivity > 850 nm/RIU. The robust design of the proposed structure is suitable for optoelectronic devices.