Optical anisotropy of organized silver nanoparticles in 2D superlattice

We compare the optical properties of spherical particles organized in a two-dimensional (2D) structure with disordered and coalesced particles. Both particle preparations were deposited on cleaved graphite substrates. When particles are arranged in a hexagonal array, the optical measurements under p-polarization show a new high-energy resonance which is interpreted as a collective effect, resulting from optical anisotropy due to the mutual interactions between particles. We support this interpretation by numerical calculations performed for finite-size clusters of silver spheres. For disordered and coalesced particles, a low-energy resonance appears instead of the high-energy resonance observed for spherical and organized particles. This is interpreted as optical shape anisotropy due to the asymmetrical shape of coalesced particles.