As outlined in the previous two chapters, the traditional understanding of antennas originates from their RF developments . Transmitting antennas are viewed as transducers that convert voltages and currents into electromagnetic waves. On the other hand, receiving antennas are viewed as transducers that convert electromagnetic waves into voltages and currents. There has been considerable attention given recently to optical antennas (see Ref.  and references therein). For instance, standard resonant antennas, i.e. ones whose characteristic length is near a multiple of a half-wavelength, such as dipoles and bowties, have been studied by many groups [33, 74]. They are ones that are the most accessible to nanofabrication processes; and, hence, their simulated properties have been experimentally verified. Dimers are another well-studied example of optical antennas specifically designed for large enhancements of local fields [37, 75, 76]. More recent examples include transitioning directive RF antennas to the optical regime, such as Yagi-Uda antennas [77, 78], arrays [79–81] and simple radiators combined with electromagnetic bandgap structures . Even more traditional schemes, such as using an antenna to excite an RF waveguide, have been extended to optical frequencies . Furthermore, nonlinear loads have been incorporated into optical antennas to control their emission properties, as well as to create harmonic generation [55, 84].
Given the intrinsic nature of the excitation of the majority of optical antennas studied to date, most RF engineers would view them simply as nano-scatterers, which have been designed to create large local fields.
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