Optical antennas (nantennas)

In response to inherently low levels of efficiency in the collection of light from photovoltaic cells, the nantenna has recently become a feature of the solar power gathering landscape. In simple terms, as this chapter illustrates, it is a conventional wire-type antenna, for transmitting or receiving electromagnetic waves, but expanded in its operational capability from the microwave and millimetre wavebands, up into the infrared and optical ranges. Unfortunately, as the chapter also emphasises, frequency scaling laws introduce significant implementation difficulties. In nantennas, the current carrying wires shrink in their cross-sectional dimensions to sizes in the nanometre range (radii less than 100 nm). In addition to the obvious fabrication problems which are encountered, even when sophisticated lithographic methods are adopted, these nanoscale dimensions impose additional limitations. The current flows in such fragile wires, enforced by the laws of physics are restricted in unexpected ways. By focusing on the dipole antenna at the nanoscale, the chapter demonstrates the negative effects, on its radiation efficiency, of enhanced field penetration into filamentary conductors and of electron kinetic effects in such wires, both of which become significant at radii of less than 100 nm. Given that in space, temperatures close to absolute zero are difficult to avoid, then for orbiting solar platforms at least, it seems possible that rectennas employing supercooled and superconducting materials could offer a route towards high efficiency light gathering systems. This new technology avenue is briefly addressed towards the end of the chapter.