Tunable Subnanometer Gap Plasmonic Metasurfaces

The index of refraction governs the flow of light through materials. At visible and near-infrared wavelengths the real part of the refractive index is limited to less than 3 for naturally occurring transparent materials, fundamentally restricting applications. Here, we carried out experiments to study the upper limit of the effective refractive index of self-assembled metasurfaces at visible and near-infrared wavelengths. The centimeter-scale metasurfaces were made of a hexagonally close packed (HCP) monolayer of gold nanospheres coated with tunable alkanethiol ligand shells, controlling the interparticle gap from 2.8 to 0.45 nm. In contrast to isolated dimer studies, the macroscale areas allow for billions of gaps to be simultaneously probed and the HCP symmetry leads to large wavelength shifts in the resonance mode, enabling subnanometer length scale mechanisms to be reproducibly measured in the far-field. We demonstrate for subnanometer gaps that the optical response of the metasurfaces agrees well with a classical (local) model, with minor nonlocal effects and no clear evidence of ligand-mediated charge transfer at optical frequencies. We determine the effective real part of the refractive index for the metasurfaces has a minimum of 1.0 for green-yellow colors, then quickly reaches a maximum of 5.0 in the reds and remains larger than 3.5 far into the near-infrared. We further show changing the terminal group and conjugation of the ligands in the metasurfaces has little effect on the optical properties. These results establish a pragmatic upper bound on the confinement of visible and near-infrared light, potentially leading to unique dispersion engineered coatings.