Optical anisotropies of metal clusters supported on a birefringent substrate

Abstract

We report on the optical properties of Ag nanoparticles supported on an insulating, birefringent substrate for nominal film thicknesses of less than 4 nm. Spectroscopic ellipsometry (SE) and reflectance-difference spectroscopy (RDS) were employed to detect out-of-plane and in-plane anisotropies of particle-plasmon resonances, respectively. Quantitative agreement between measured and calculated spectra is achieved using the morphological parameters determined independently from electron microscopy images. The substrate birefringence allows the detection of cluster layers by RDS via the screening of the substrate signal proportional to the layer susceptibility. In addition, an intrinsic lateral anisotropy, which is explained by the lifting of the degeneracy of planar modes due to the anisotropic images forming in the substrate, has been detected. For higher coverages the influence of anisotropic cluster shapes and strong multipolar resonances are identified.