Plasmonic Metasurfaces with Tunable Gap and Collective Surface Plasmon Resonance Modes

Optical properties of a plasmonic metasurface made of a monolayer of gold nanoparticles in close proximity to an aluminum thin film were studied numerically and experimentally. Extinction spectra of the plasmonic metasurface were studied as functions of the thickness of a dielectric spacer between the monolayer of gold nanoparticles and the aluminum film in the visible wavelength range. The goal was to understand the excitation of a collective surface plasmon resonance (SPR) mode and a gap plasmon mode as well as their dependence on the spacer thickness, nanoparticles spacing, and their size. By using finite-difference-time-domain (FDTD) calculations, we find that the SPR extinction peak first red-shifts and then splits into two peaks. The first extinction peak is associated with the collective SPR mode of the monolayer, and it shifts to shorter wavelengths as the spacer layer decreases. As the spacer layer decreases from 35 to 7.5 nm, the second peak gradually appears in the extinction spectra of the metasurface. We assign the second peak to the gap mode. The gap mode first appears at around 620 nm or greater, and it shifts to larger wavelength for larger nanoparticle spacing and size. The FDTD simulations are confirmed by an experimental examination of the dispersion curves of a similar multilayer system. The computational results match the experimental results and confirm the excitation of the two modes.