Optical nonlinearities of Au/SiO2 composite thin films prepared by a sputtering method

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Abstract
Third‐order nonlinear optical properties of Au/SiO2 composite thin films have been investigated by means of a degenerate four‐wave mixing at room temperature. In the optical‐absorption spectra the absorption peak due to the surface plasmon resonance of Au particles is observed at the wavelength around 530 nm. With increasing the mean diameter of Au particles, the absorption at the peak is increased and the full width at half‐maximum of the absorption band is decreased from 130 to 80 nm. The third‐order nonlinear susceptibility χ(3) exhibits a peak at the wavelength of the absorption peak and the maximum value of χ(3) is obtained to be 2.0×10−7 esu. The size‐dependent enhancement of the χ(3) in Au particles with the mean diameter of 3.0–33.7 nm has been also investigated. The value of χ(3)/α (α: absorption coefficient) for the films is increased upon an increase of mean diameter of Au particles. This is explained by the size dependences of the local‐field factor and the imaginary part of dielectric constant of the metal particles.