Abstract
<jats:p>Dense assemblies of metal nanoparticles offer a scalable route to high‑index optical metamaterials, yet their collective response and loss mechanisms remain poorly understood. Here, we synthesize quasi‑spherical Ag nanoparticles with diameters ranging from 17 to 66 nm and assemble them into close‑packed monolayers with controlled interparticle gaps. These films exhibit size‑dependent refractive indices ranging from 4.4 to 10.3, with high index and low loss emerging on the red shoulder of the coupled dipolar resonance. We introduce a compact analytical model in which the collective mode is an LC‑type oscillation governed by gap capacitance, f‑sum‑rule‑limited oscillator strength, three loss channels, and polydispersity broadening. The model quantitatively reproduces the measured optical constants and identifies the interparticle gap as the dominant lever controlling the achievable index and loss figure of merit. We compute Bergman-Milton fundamental limits, finding that our best assemblies reach 72% of a high-index–low-loss Pareto frontier for silver/air composites, with the remaining samples approaching within a factor of 2–4. Together, experiment and theory establish silver nanoparticle assemblies as a tunable platform approaching fundamental bounds and define clear pathways for further performance gains.</jats:p>