The field of optical nanoparticles, a rapidly developing area of optics, is reviewed. The basic concept of an optical antenna is formulated and the main characteristics relevant to this structure are identified. A classification of nanoantenas in metallic and dielectrics is carried out (the latter including nanoantenas of semiconductors). For each category, the literature is reviewed and the strengths and weaknesses of the different approaches are discussed. The fundamentals of non-linear optical antennas are described.
Nowadays, it is common to refer to an electromagnetic transmitter or receiver as an antenna. They have been developed since the beginning of electromagnetism and the need to increase the bandwidth of the communication link has fueled this development. However, the development of antennas that support a shorter and shorter wavelength did not reach the optical regime. One reason is that before the antennas could reach optical frequencies reliably, optical fibers had eliminated the need to develop optical antennas for telecommunications applications. Although antennas are a key element for many devices in the radio or microwave system, its optical analogue is almost non-existent in today's technology. In contrast, optical radiation is manipulated by redirecting wave fronts with lenses and mirrors. This type of manipulation is based on the wave nature of the electromagnetic waves and is therefore not suitable for controlling the field in the sub-wavelength scale (the diffraction limit).