For radio engineers it is a common task to combine several antennas to form an antenna array. This gives them several degrees of freedom for shaping the radiation pattern according to their needs. By selecting different types of individual elements, their relative position in space, their respective orientation, and the amplitude and phase of the induced currents, one can engineer the radiated beam properties . In the new research field of optical nanoantennas, the possibilities of arraying antennas have hardly been explored yet. This is mainly due to the challenges in fabricating and driving the arrays, as well as the yet limited possibilities of characterization. Nevertheless, application of RF antenna array concepts into optical regimes promises tremendous technological advances: increasing the directivity and gain aids in distant signal transmission and reception (similarly to the concepts used in satellite communication), coupling nanoemitters and nanoreceivers to antenna arrays enhances their efficiency with the potential of bridging the size gap between optical radiation and subwavelength emitters or detectors and employing phase retarders allows for steering of optical beams.
In this chapter, we introduce the concepts of array theory and scale them to optical frequencies. We start with a short introduction on RF antenna array theory and discuss the differences that have to be accounted for at optical frequencies. Subsequently, the possibility of beam shaping at optical frequencies is discussed. Numerical and experimental studies on a closely spaced 1D array of plasmonic dipole antennas, whose design is analogous to the well-known RF Yagi–Uda antenna , give insight into the dynamics of the optical modes that are supported by the antenna structure.