Optical Antennas: Controlling Electromagnetic Scattering, Radiation, and Emission at the Nanoscale

The emerging field of optical nanoantennas has been extending the reach of traditional antenna technology to the realms of photonics, nanotechnology, and quantum electrodynamics. In this article, we review the basic concepts of nanoantennas while highlighting similarities and differences with their low-frequency counterparts. We discuss how several concepts from traditional antenna technology, e.g., the antenna input impedance, can be translated to optical frequencies, and examine the new phenomena and engineering opportunities enabled by the material platform available at these frequencies. In this article, particular attention is devoted to different kinds of plasmonic nanoantennas (nanodipoles, nanoloops, and nanopatches) and to systematic approaches to control and enhance their performances, especially for impedance matching and directivity enhancement. We also show how linear and nonl inear plasmonic nanoantennas represent an ideal platform to realize anomalous and extreme forms of classical and quantum light-matter interactions, including artificial optical magnetism, spontaneous emission enhancement, and anomalous coupling between quantum emitters. As antenna concepts are increasingly appreciated and used in the design of nanophotonic devices and systems, we believe that the exciting field of optical antennas holds promise for large technological and scientific impact in the coming years.