Antennas for light

nature photonics | VOL 5 | FEBRUARY 2011 | www.nature.com/naturephotonics 83 Optical antennas, analogues of microwave and radiowave antennas, are a new concept in physical optics1. They are an enabling technology for manipulating and controlling optical radiation at subwavelength scales2. Optical antennas are the subject of a growing number of scientific studies, and hold promise for enhancing the efficiency of photodetection3,4, light emission5,6, sensing7, heat transfer8,9 and spectroscopy10. Traditionally, the field of optics and photonics deals with the control of propagating optical radiation using elements such as mirrors, lenses, fibres and diffractive elements. On the other hand, in the radiowave and microwave regime, using antennas of various designs to control electromagnetic fields on the subwavelength scale is a well-established technique. Antennas are pervasive in almost all areas of modern technology, ranging from satellite communications to baby phones. Despite the widespread use of radiowave and microwave antennas, the optical analogue has not yet made its debut in technological applications, primarily because of its small size requirement. The characteristic dimensions of an antenna are of the order of the radiation wavelength, and for optical antennas this requires fabrication accuracies down to a few nanometres. However, this length scale has become increasingly accessible as the tools of nanoscience and nanotechnology have improved. Prototype optical antennas have been fabricated by top-down nanofabrication tools such as focused ion beam milling11,12 or electron-beam lithography13,14, and also by bottom-up self-assembly schemes15,16. It is evident that the fabrication of optical antenna structures provides an emerging opportunity for realizing new optoelectronic devices. The objective of optical antenna design is equivalent to that of classical antenna design: to optimize the energy transfer between a localized source or receiver and the free-radiation field (Fig. 1). In certain recent studies, however, the term ‘optical antenna’ has clearly been stretched beyond its common definition in radiowave technology. An antenna is not simply a resonator or a strong scatterer. Instead, it is defined by its function, namely as a transducer between free radiation and localized energy. Its efficiency is defined by the degree of localization and the magnitude of transduced energy. The term ‘antenna’ is used in a wide range of contexts, such as for tent posts, the beams of sailing boats or insect whiskers. The electromagnetic antenna, originally referred to as an ‘aerial’, is a transducer between electromagnetic waves and electric currents, and generally operates in the radiofrequency regime. In analogy with the electromagnetic antenna, we define the optical antenna as a device that antennas for light