Optical response of ZnO films embedded with randomly dispersed Au nanoparticles

The interaction of electromagnetic radiation with metals of sub-wavelength dimensions manifests in localized surface plasmon resonance (LSPR), an anomalous phenomena unique to metal nanoparticles-dielectric composite films. The talk focuses on the optical response of two-component composites consisting of Au nanoparticles (AuNPs) in thin dc magnetron sputtered ZnO films, and on the computation of their effective dielectric function. Because of the randomness associated with the shape, size and inter-particle separation of AuNPs in the composite media, direct evaluation of optical constants by simple mixing formula consisting of a trivial average of dielectric function of constituents and volume fractions is difficult. The spectral density representation of Bergman-Milton formulation has been used to analyze the transmittance spectra and to extract the effective dielectric functions of AuNPs-ZnO films. A correlation between the spectral density function of the composites and the morphology of AuNPs has been drawn by systematically studying the inclusions’ size dependence transmittance spectra, and henceforth useful information in the context of microstructure of Ag nanoparticles (AgNPs) embedded in ZnO films have been retrieved from spectral density function of AgNPs-ZnO composites. It will be shown that by reducing the thickness of the ZnO film that separates the AuNPs-ZnO composites in ZnO/AuNPs-ZnO/ZnO/AuNPs-ZnO/ZnO/GP structure (GP: glass plate), it is possible to increase the number density of smaller size AuNPs and hence achieve a sharp dipole-dominated LSPR feature.