High Refractive Index Silver Nanoparticle Metamaterials Approaching Fundamental Bounds

Dense assemblies of metal nanoparticles offer a scalable route to high-index optical metamaterials, yet their collective response and loss mechanisms remain poorly understood. We synthesize quasi-spherical Ag nanoparticles with diameters ranging from 17 to 66 nm and assemble them into closely-packed monolayers. These films exhibit size-dependent refractive indices reaching a maximum of 10.3 with a low loss region emerging on the red shoulder of the resonance. We introduce a compact analytical model that quantitatively reproduces the measured optical constants and identifies the interparticle gap as the lever controlling the achievable index and loss. We compute Bergman-Milton fundamental limits, finding that our best assemblies reach two-thirds of the high-index-low-loss Pareto frontier for silver/polystyrene composites of arbitrary microstructure with the remaining samples approaching within a factor of 1.7-4. Together, experiment and theory establish silver nanoparticle assemblies as a tunable platform approaching fundamental bounds and define pathways for further performance gains.