Fabrication and optical properties of nanostructured plasmonic Al₂O₃/Au-Al₂O₃/Al₂O₃ metamaterials

Discontinuous multilayer (DML) thin films, which consist of nano-granular metals (NGMs) embedded in a dielectric matrix, have attracted significant interest as engineered plasmonic metamaterials. In this study, a systematic layer-by-layer deposition of three-dimensional sub-wavelength periodic plasmonic DML structures via the radio frequency sputtering of a composite target has been reported. The overall optical response of the DML films composed of Au-Al₂O₃ NGM homogenous layers, which are periodically sandwiched between two amorphous Al₂O₃ layers, are studied using reflection spectroscopic ellipsometry and transmission spectroscopy techniques. By applying the analytical optical approaches based on multiple Gaussian oscillators, ambient DML sub-wavelength structures have been successfully modeled. As a result, the effects of the size and shape of the Au nanoparticles as well as of the surrounding and interfacial media on their localized surface plasmon resonance (LSPR) are elucidated, and the related films thickness and effective optical constants are determined. Interestingly, during the examination of resonance frequencies and dielectric functions, the obtained DML structures exhibit unusual characteristics that are different from those of their NGM constituents due to the electromagnetic interactions of the NGM layers with the LSPR, which represent metamaterial features.