engleski

Tuning the effective dielectric function of thin film metal-dielectric composites by controlling the deposition

Metal dielectric composites have been in the focus of interest due to the surface plasmon resonance phenomena and its large number of related applications, ranging from biological and chemical sensing to telecommunications. Recently, novel photonic applications of metal-dielectric composites have been devised, in fields such as metamaterials operating in the optical range or wavelength-tunable superlenses. The key element is the possibility to broadly tailor the effective dielectric function of these composites, according to the predictions of effective medium theories. In the present work we study the effective dielectric function of Ag-SiO2 composites in thin film phase deposited by electron beam evaporation. The composites are obtained by the subsequent deposition of a small amount of Ag and SiO2 on a glass substrate. Due to the typical island-like growth of Ag on dielectrics, the metal film is composed of clusters of nanometric dimensions rather than being a compact film. The effective dielectric function of the resulting composite is determined by spectroscopic ellipsometry measurements. It is shown that by varying the substrate temperature only, the effective dielectric function of the composites can be widely modified. Composites deposited on heated substrates are characterized by a narrow localized surface plasmon absorption band, while those deposited at room temperature show a broader plasmon resonance and additional absorption in the infrared. This optical behavior can be associated with the growth of metal film either as isolated nearly spherical particles or to non-spherical particles with some degree of interconnectivity. The description of the effective dielectric function using classical effective medium theories fails and the experimental results can be explained only in the more general framework of the Bergman spectral density theory. Overall, the fabricated composites prove that a widely tunable effective dielectric function can be achieved by controlling the deposition temperature only. However, a correct quantitative modeling requires using advanced effective medium concepts.

surface plasmon; metal islands; effective medium theory

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