Sodium-Based Surface Plasmon Resonances for High-Performance Optical Sensing in the Near Infrared

Surface plasmon resonance (SPR) owing to its extreme sensitivity to the refractive index of surrounding medium has been widely applied to chemical and biological sensing. However, it is difficult to realize SPR sensing simultaneously with large sensitivity and figure of merit (FOM) in a conventional prism coupling structure. In this paper, we numerically study the SPRs in a polymer-protected sodium thin film and sinusoidal nanograting based on a prism Kretschmann configuration using finite element method (FEM). In the near-infrared wavelengths, both angular and wavelength interrogation are investigated and high sensitivity (14.9 μm/RIU and 750 deg/RIU for the thin film and nanograting, respectively) and FOM (480.6 /RIU and 255.6 /RIU for the thin film and nanograting, respectively) can be achieved simultaneously. It should be noticed that the large FOM mainly originates from the narrow resonant linewidth. Additionally, our nanograting structure can support a self-reference sensing scheme, which is beneficial to realize accurate sensing. Our FEM simulations are in good agreement with the calculation by the transfer matrix method (TMM) and rigorous coupled wave analysis (RCWA). Our proposed sodium-based SPR structures do not require complex plasmonic nanostructures and are compatible with a thermo-assisted spin-coating process, which shows great potential in realizing high-performance optical sensing beyond noble metals and expands the application scope of sodium-based plasmonic devices.