Bottom-up fabrication and optical characterization of dense films of meta-atoms made of core-shell plasmonic nanoparticles.

In an attempt to fabricate low index metamaterials by a bottom-up approach, meta-atoms constituted of silica-coated silver nanoparticles are assembled by a Langmuir-Schaefer technique into thin films of large area and well-controlled thickness. The silica shells ensure a constant distance between the silver cores, hence providing a constant coupling of the localized surface plasmon resonance (LSPR) of the nanoparticles in the assembled composite material. The optical response is studied by normal angle spectral reflectance measurements and by variable angle spectroscopic ellipsometry. The normal incidence data are described well in the framework of a single effective Lorentz oscillator model. The resonance of the assembled material is blue-shifted and shows no significant broadening with respect to the absorption band of the individual nanoparticles. The observation of these two effects is enabled by the core-shell structure of the meta-atoms that prevents aggregation of the metallic cores. The ellipsometry study confirms the general behavior and reveals the natural birefringence of the few-layer materials. The amplitude of the observed resonance is weaker than expected from the Maxwell-Garnett mixing rule. This well-characterized system may constitute a good model for numerical simulations.