Near-infrared narrow plasmonic resonances for high-performance optical sensing in a sodium-based nanograting

Plasmonic resonances are attractive and promising for biosensing applications benefiting from their strong nanoscale light confinement and sensitivity to the refractive index of surrounding medium. Here, we numerically and theoretically investigate a metal-insulator-metal (MIM) nanograting structure, where gold nanostrip arrays are placed on a thin dielectric layer with a sodium film underlay. The structure is obliquely excited by a transverse magnetic (TM) wave from a prism supporting the whole MIM structure. We found that the plasmonic surface lattice resonance (SLR) mode and gap plasmon resonance (GPR) mode can be efficiently excited in our structure at the near infrared wavelength. Remarkably, the SLR mode can achieve ultranarrow linewidth less than 3 nm (quality factor > 500), sensitivity over 1000 nm/RIU and the corresponding figure of merit (FOM) larger than 300 RIU-1. Our study may pay a way to achieve high-performance sensing in a simple plasmonic nanostructure that could be vastly used in biological and chemical circumstances.