An optical antenna is a mesoscopic structure that enhances the local light-matter interaction. Similarly to their radiowave analogs, optical antennas mediate the information and energy transfer between the free radiation field and a localized receiver or transmitter. The degree of localization and the magnitude of transduced energy indicate how good an antenna is. We thus define an optical antenna as a device designed to efficiently convert freepropagating optical radiation to localized energy, and vice versa . In this sense, even a standard lens is an antenna, but since the degree of localization is limited by diffraction, the lens is a poor antenna. To characterize the quality and the properties of an antenna, radio engineers have introduced antenna parameters, such as gain and directivity. Optical antennas hold promise for controllably enhancing the performance and efficiency of optoelectronic devices, such as photodetectors, light emitters, and sensors.
Although many of the properties and parameters of optical antennas are similar to those of their radiowave and microwave conuterparts, there are important differences resulting from their small size and the plasmon resonances of metal nanostructures. In this chapter we introduce the basic principles of optical antennas, building on the background of both radiowave antenna engineering and plasmonics.
Significance of optical antennas
The length scale of free radiation is determined by the wavelength λ, which is on the order of 500 nm. However, the characteristic size of the source generating this radiation is significantly smaller, typically sub-nanometer.