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. * jake.fontana@nrl.navy.mil

The emergence of metamaterials (MMs) has led to groundbreaking photo-physical phenomena, which arise from their novel structure-dependent properties. Consisting of “meta-atom” building blocks, MMs can be organized into subwavelength metal/dielectric structures using bottom-up or top-down nanofabrication techniques. Optical metal metasurfaces are a class of MMs with macroscopic lateral dimensions but composed of one to few subwavelength layers of precisely oriented metal-based elements over a large surface area. In this review, we focus on gold metasurfaces, highlighting their fabrication methods, morphological characterization, as well as linear and nonlinear optical properties. We then review our recent work on fabricating and characterizing self-assembled gold metasurface. An interesting characteristics of the gold metasurfaces is their wide range of linear refractive indices, varying from n0 ~ 0.5 in the visible to n0 ~ 4 in the short wavelength infrared spectral region. Third-order nonlinearities are characterized by the Z-scan technique at wavelengths on- and off-plasmonic resonance of the gold metasurface. Experimental results on the relationship between the third-order nonlinearity of the self-assembled gold metasurface as a function of the linear response are presented for the first time. We conclude by discussing the potential applications and future outlook of self-assembled plasmonic metasurfaces.