Image resolution of surface-plasmon-mediated near-field focusing with planar metal films in three dimensions using finite-linewidth dipole sources

We study the role of surface plasmons in the near-field focusing of a finite-linewidth point dipole by a planar silver film using three-dimensional finite element calculations. We find that the intensity distribution in the image plane at a distance of 60 nm from the source is narrowed by a factor of 1.4 in the presence of a 30-nm-thick silver film. The lateral field components are found to be focused significantly better. We show that the difference is caused by unavoidable stray fields normal to the lens surface due to the interface charge distribution induced by the presence of surface plasmons.