Antenna ideas at the nanoscale: Single quantum emitters at visible wavelengths controlled using plasmonics and metamaterials

Current research in optics, microscopy and quantum information processing frequently deals with the interaction of single quanta of light with single emitters, such as single molecules, quantum dots, or luminescent defect centers in solid materials such as silicon and diamond. Since it is impossible to focus a light wave to below the diffraction limit, i.e., to a size close to the cross section of a single emitter, and since single emitters tend to emit omnidirectionaly it is very difficult to interface single emitters and single photons with unit efficiency. There is hence intense research into photonic strategies that shape electromagnetic fields in such a way that unit efficiency interaction between light and matter is possible. On the one hand high quality factor ultrasmall dielectric cavities are pursued in solid state quantum optics already for a long time. On the other hand, very recently interest in socalled `optical antennas´ has surged. Such devices are inspired by radio wave antennas, yet scaled down by a factor 10<sup>6</sup> to operate at optical wavelengths. Since the intrinsic response of metals is very different at such high frequencies, these antennas employ socalled `plasmon resonances´ to enhance their cross sections for interaction with light. In this contribution we introduce plasmon resonances and their role in current nano-optics research.