From Nano‐ to Picocavities: The Role of Atomistic Features

Understanding how atomistic structural features reshape plasmonic properties is essential for the rational design of nanostructures for enhancing light‐matter interactions at the nanoscale. Here, we investigate the optical response of realistic Ag and Au nanocavities by means of the fully atomistic frequency‐dependent fluctuating charges and fluctuating dipoles ( ω FQF μ ) approach, which enables the description of realistic systems while also accounting for quantum tunneling effects. We compare nanocavities formed by smooth tip‐substrate junctions with picocavities generated by atomistically defined needles. We further push the analysis to the ultimate atomistic limit by considering a picocavity formed by a single adatom. By combining absorption spectra, induced charge densities, electric field maps, and effective localization areas, we show that atomistic details strongly affect plasmon modes, field confinement, and enhancement. Remarkably, Ag and Au nanostructures display similar but distinct plasmonic behaviors, reflecting the different intrinsic electronic responses of the two metals. These results establish a direct structure‐property relation between atomic‐scale morphology and plasmon localization, providing physical insight for the design of plasmonic cavities with tailored near‐field properties.