Optical properties of Au–TiO2 and Au–SiO2 granular metal thin films studied by Spectroscopic Ellipsometry

Abstract

We report on the optical properties in the dielectric regime of gold nanostructured granular thin films fabricated through sputter deposition with a composite target at room temperature and over a wide photon energy range (0.62–4.13eV) by means of Spectroscopic Ellipsometry. The thickness and the films effective optical constants are successfully determined using an approach based on multiple Gaussian oscillators. In the quasi-static regime, i.e., 2R≪λ, and in the dipole approximation, examining the real and imaginary parts, ε1, ε2, of the dielectric function, it is shown that the dc optical conductivity is almost negligible (σ=ωε0ε2≪10−5Ωcm−1) and only the capacitive contribution holds for the electron-phonon relaxation in localized surface plasmon of the gold particles. Furthermore, we find that the resonant frequencies ωp becomes red-shifted when the particles are electromagnetically coupled to each other or when the surrounding medium dielectric constant, εm, increases, thus exhibiting a wide spectral tuning range of 1.95–2.24eV.