The concept of nanoantennas has emerged in optics as an enabling technology for controlling the spatial distribution of light on subdiffraction length scales. Analogously to classical antenna design, the objective of optical antenna design is the optimization and control of the energy transfer between a localized source, acting as receiver or transmitter, and the free radiation field. Most of the implemented optical antenna designs operate in the linear regime that is, the radiation field and the polarization currents are linearly dependent on each other. When this linear dependence breaks down, however, new interesting phenomena arise, such as frequency conversion, switching and modulation. Beyond the ability of mediating between localized and propagating fields, a nonlinear optical antenna provides the additional ability to control the interaction between the two. Figure 8.1 sketches an example where the nonlinear antenna converts the frequency of the incident radiation, thus shifting the frequency of a signal centered at ω1 by a predefined amount Δω into a new frequency band centered at ω2. Here we review the basic properties of nonlinear antennas and then focus on the nonlinearities achievable in either single-NP systems or more complex coupled-NP systems. In practice, the use of nonlinear materials – either metals or dielectrics – in the design of optical antennas is a promising route towards the generation and control of optical information.
The study of nonlinear optical antennas is still in its infancy. The design principles are based on the well-established field of nonlinear optics [295, 296] that has its origins in the early 1960s, when SHG was first observed in a piezoelectric crystal .