  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 macro-scale 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
