World's first optical ‘rectenna' uses aluminium coated carbon nanotubes

Scientists in the US have assembled the first ever optical rectenna - a device that’s part antenna, part rectifier diode (containing calcium and aluminium), and uses the functions of both to convert light into a direct electrical current.

The device uses an array of aluminium oxide coated carbon nanotubes that function like tiny antennas to capture light, and from this it generates an oscillating charge that travels through the rectifier, which then switches on and converts the alternating current (AC) into a direct current (DC). It’s taken scientists more than 40 years to get light working in this way, and the invention could lead to new technologies for converting heat waste into electricity and more efficiently capturing solar energy - without the need for cooling.

"A rectenna is basically an antenna coupled to a diode, but when you move into the optical spectrum, that usually means a nanoscale antenna coupled to a metal-insulator-metal diode," one of the team, Baratunde Cola from the Georgia Institute of Technology, said in a press release. "The closer you can get the antenna to the diode, the more efficient it is. So the ideal structure uses the antenna as one of the metals in the diode - which is the structure we made."

The process for making an optical rectenna is rather simple from a scientist's point of view. One needs any type of conductive substrate, and on top of this, one needs to grow vertically-aligned ‘forests’ of microscopic carbon nanotubes in a carpet-like arrangement. These nanotubes are then coated in an aluminium oxide electrical insulator, and several optically-transparent, thin layers of calcium and aluminium are placed on top to act as the anode.

This metal-insulator-metal diode structure is the fastest diode in the world, capable of switching on and off at record-high petahertz speeds (1 quadrillion times per second, or 1 million times faster than a GHz). This forces the electrons generated by the antenna to flow one direction into the top electrode to create a small direct current.

Because optical rectennas are built from such minuscule components, an array of them will be required to produce a significant current, and right now the efficiency of such a device sits at just below 1 percent. This means very little light that actually hits the device gets converted into electricity.

The team’s next big challenge is figuring out how to increase this, but they say they're confident that they can get this percentage to at least 40, and hope to achieve commercial potential within a year.

"We could ultimately make solar cells that are twice as efficient at a cost that is 10 times lower, and that is to me an opportunity to change the world in a very big way," Cola says in the press release. "As a robust, high-temperature detector, these rectennas could be a completely disruptive technology if we can get to one percent efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture."