Probing the out-of-plane optical response of plasmonic nanostructures using spectroscopic ellipsometry

A simplified approach to investigate the out-of-plane response of plasmonic nanostructures using spectroscopic ellipsometry (SE) is presented. One-dimensional self-assembled arrays of Ag nanoparticles (NP's) were grown on stepped Al2O3(0001), in ultrahigh vacuum, using deposition at a glancing angle. The SE response was measured with the plane of incidence aligned along, and across, the surface steps. From the raw data, an anisotropic surface excess function (ASEF) can be extracted, whose properties depend only on the dielectric function of the NP layer. Three resonances are clearly seen in the ASEF: two in-plane resonances, which correspond to the resonances measured using normal incidence reflection anisotropy spectroscopy, and the out-of-plane resonance. A dipole model is used to simulate the optical response of the NP layer, where the presence of the out-of-plane resonance provides an important additional constraint in developing the model.