Role of surface-to-volume ratio of metal nanoparticles in optical properties of Cu:Al2O3 nanocomposite films

We report on the role of the surface-to-volume ratio of Cu nanoparticles (NPs) both in the linear and nonlinear optical properties of Cu:Al2O3 nanocomposite films. The results show that when the shape of the NPs deviates sufficiently from that of a sphere, the increase of the fraction of metal atoms present at the surface (NS) with respect to the total amount of atoms (NT) in the NP leads to a substantial reduction of the enhancement of the local field. As a consequence, for NS∕NT values above a certain threshold (≈0.4–0.5), the surface–plasma resonance is smeared out and the nonlinear optical response of the nanocomposite film becomes very weak and independent of the dimensions of the NPs or their volume fraction in the matrix.