Near Field Investigation of a Plasmonic Lüneburg Lens

In this work, we investigate the interaction of surface plasmons with a plasmonic Lüneburg lens using near field scanning optical microscopy. Gray-scale electron beam lithography is used to prepare a dome-shaped resist structure on top of a gold film. This particular shape yields the effective refractive index profile of a Lüneburg lens for surface plasmons propagating at the film surface at an energy of ℏω=1.72\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\hbar \omega ={1.72}$\end{document} eV. Next to the Lüneburg lens a grating coupler is milled into the gold film with focused ion beam. The surface plasmons are launched to propagate through the lens and the near field pattern is scanned. We clearly identify a focal spot in the near field signal at the outer perimeter of the lens. In addition, we observe a beating pattern arising from further plasmon waves excited by higher orders of the grating coupler. The emergence of this beating pattern allows the detection of the plasmon’s wave fronts. An analytical model was used to retrieve the properties of the participating wave components. The measured near-field pattern could be modeled very well with electromagnetic simulations applying the effective refractive index approach.