A nantenna (nano antenna) is a nanoscopic rectifying antenna, an experimental technology being developed to convert light to electric power. The concept is based on the rectenna (rectifying antenna), a device used in wireless power transmission. A rectenna is a specialized radio antenna which is used to convert radio waves into direct current electricity. Light is composed of electromagnetic waves like radio waves, but of much smaller wavelength. A nantenna is a very small rectenna the size of a light wave, fabricated using nanotechnology, which acts as an “antenna” for light, converting light into electricity. It is hoped that arrays of nantennas could be an efficient means of converting sunlight into electric power, producing solar power more efficiently than conventional solar cells. The idea was first proposed by Robert L. Bailey in 1972. As of 2012, only a few nantenna devices have been built, demonstrating only that energy conversion is possible. It is unknown if they will ever be as cost-effective as photovoltaic cells.
A nantenna is designed to absorb specific wavelengths that are proportional to the size of the nantenna. Currently, Idaho National Laboratories has designed a nantenna to absorb wavelengths in the range of 3–15 μm. These wavelengths correspond to photon energies of 0.08 – 0.4 eV. Based on antenna theory, a nantenna can absorb any wavelength of light efficiently provided that the size of the nantenna is optimized for that specific wavelength. Ideally, nantennas would be used to absorb light at wavelengths between 0.4 and 1.6 μm because these wavelengths have higher energy than far-infrared (longer wavelengths) and make up about 85% of the solar radiation spectrum.
Incident light on the antenna causes electrons in the antenna to move back and forth at the same frequency as the incoming light. This is caused by the oscillating electric field of the incoming electromagnetic wave. The movement of electrons is an alternating current in the antenna circuit. To convert this into direct current, the AC must be rectified, which is typically done with some kind of diode. The resulting DC current can then be used to power an external load. The resonant frequency of antennas (frequency which results in lowest impedance and thus highest efficiency) scales linearly with the physical dimensions of the antenna according to simple microwave antenna theory. The wavelengths in the solar spectrum range from approximately 0.3-2.0 μm.
Thus, in order for a rectifying antenna to be an efficient electromagnetic collector in the solar spectrum, it needs to be on the order of hundreds of nm in size.
The most apparent advantage nantennas have over semiconductor photovoltaics is that nantenna arrays can be designed to absorb any frequency of light. The resonant frequency of a nantenna can be selected by varying its length. This is an advantage over semiconductor photovoltaics, because in order to absorb different wavelengths of light, different band gaps are needed. In order to vary the band gap, the semiconductor must be alloyed or a different semiconductor must be used altogether.