Optical rectenna
[1] A rectenna is a circuit containing an antenna and a diode, which turns electromagnetic waves into direct current electricity.
One challenge is that light has such a high frequency—hundreds of terahertz for visible light—that only a few types of specialized diodes can switch quickly enough to rectify it.
Due to these and other challenges, optical rectennas have so far been restricted to laboratory demonstrations, typically with intense focused laser light producing a tiny but measurable amount of power.
[4] In 2008 it was reported that Idaho National Laboratories designed an optical antenna to absorb wavelengths in the range of 3–15 μm.
Robert Bailey, along with James C. Fletcher, received a patent (US 3760257 ) in 1973 for an "electromagnetic wave energy converter".
In 1974, T. Gustafson and coauthors demonstrated that these types of devices could rectify even visible light to DC current[7] Alvin M. Marks received a patent in 1984 for a device explicitly stating the use of sub-micron antennas for the direct conversion of light power to electrical power.
[9] In 2002, ITN Energy Systems, Inc. published a report on their work on optical antennas coupled with high frequency diodes.
[10] Vertical arrays of multiwall carbon nanotubes (MWCNTs) grown on a metal-coated substrates were coated with insulating aluminum oxide and altogether capped with a metal electrode layer.
Due to the small diameter of MWCNT tips, this combination forms a diode that is capable of rectifying the high frequency optical radiation.
Cola and his team later solved the challenges with device instability by modifying the diode structure with multiple layers of oxide.
Future efforts will be focussed on improving the device efficiency by investigating alternative materials, manipulating the MWCNTs and the insulating layers to encourage conduction at the interface, and reduce resistances within the structure.
Because of simplifications used in typical rectifying antenna theory, there are several complications that arise when discussing optical rectennas.
From a purely device perspective, the I-V characteristics would appear to no longer be ohmic, even though Ohm's law, in its generalized vector form, is still valid.
[6] The ideal wavelengths of 0.4–1.6 μm correspond to frequencies of approximately 190–750 THz, which is much larger than the capabilities of typical diodes.
When compared to the theoretical efficiency of single junction solar cells (30%), optical rectennas appear to have a significant advantage.
The assumptions involved in the rectenna calculation are based on the application of the Carnot efficiency of solar collectors.
The high frequency of light in the ideal range of wavelengths makes the use of typical Schottky diodes impractical.
Although MIM diodes show promising features for use in optical rectennas, more advances are necessary to operate efficiently at higher frequencies.
In moving up to a greater production scale, laboratory processing steps such as the use of electron beam lithography are slow and expensive.
The master template fabricated by Idaho National Laboratories consists of approximately 10 billion antenna elements on an 8-inch round silicon wafer.
Researchers at the University of Connecticut are using a technique called selective area atomic layer deposition that is capable of producing them reliably and at industrial scales.
His prototype was a 30 x 61 cm of plastic, which contained only 0.60 USD of gold in 2008, with the possibility of downgrading to a material such as aluminum, copper, or silver.
An assessment for solar energy collection found that, to get high efficiency, the diode would need a (dark) current much lower than 1μA at 1V reverse bias.
To improve the efficiency of carbon nanotube-based rectenna: Researchers currently hope to create a rectifier which can convert around 50% of the antenna's absorption into energy.
Future goals will be to attempt to manufacture devices on pliable substrates to create flexible solar cells.
