Introduction

Nano-optical devices have a great potential for technological applications [201, 597, 598]. Consequently, the investigation of plasmonic excitations in nanostructures and on surfaces has evolved into a tremendous research field, made possible only by the progress in nanotechnology. Nowadays, nanoantennas with highly complex shapes are fabricated with an extremely high accuracy by standardized procedures [564]. The spectral features and near-field properties of such optical antennas are determined on a length scale that is intrinsically smaller than the diffraction limit of electromagnetic waves. However, experimental access to the spatial properties of these antennas on the nanoscale is essential for an understanding of the underlying mechanisms that lead to strong near-field enhancements, interferences and mode hybridization. Thus, there is a particular need for a real-space microscopy technique that delivers information about near-field distribution within and in the vicinity of nanostructures, with a resolution below the diffraction limit. In addition to pure imaging of static field distributions, knowledge of the dynamical properties of electronic excitations is relevant for encoding and manipulation of information on the nanoscale. The microscopic understanding of the associated dynamics is crucial for many other research fields, such as molecular biology or catalytic chemistry. Considering technological applications, well-tuned spectral properties and high reproducibility of the nanostructures is most important. Smallest differences on the nanoscale of individual structures (e.g. induced by the fabrication process) lead to strong variations of their optical response.