First optical rectenna—combined rectifier and antenna—converts light to DC current

September 28, 2015
A carbon nanotube optical rectenna converts green laser light to electricity in the laboratory of Baratunde Cola at the Georgia Institute of Technology. Credit: Rob Felt, Georgia Tech

Using nanometer-scale components, researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.

Based on multiwall carbon nanotubes and tiny rectifiers fabricated onto them, the optical rectennas could provide a new technology for photodetectors that would operate without the need for cooling, energy harvesters that would convert waste heat to electricity - and ultimately for a new way to efficiently capture .

In the new devices, developed by engineers at the Georgia Institute of Technology, the carbon nanotubes act as antennas to capture light from the sun or other sources. As the waves of light hit the nanotube antennas, they create an oscillating charge that moves through rectifier devices attached to them. The rectifiers switch on and off at record high petahertz speeds, creating a small direct current.

Billions of rectennas in an array can produce significant current, though the efficiency of the devices demonstrated so far remains below one percent. The researchers hope to boost that output through optimization techniques, and believe that a rectenna with commercial potential may be available within a year.

"We could ultimately make solar cells that are twice as efficient at a cost that is ten times lower, and that is to me an opportunity to change the world in a very big way" said Baratunde Cola, an associate professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech. "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."

The research, supported by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center and the Army Research Office (ARO), is scheduled to be reported September 28 in the journal Nature Nanotechnology.

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Using nanometer-scale components, researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current. Credit: Georgia Tech

Developed in the 1960s and 1970s, rectennas have operated at wavelengths as short as ten microns, but for more than 40 years researchers have been attempting to make devices at . There were many challenges: making the antennas small enough to couple optical wavelengths, and fabricating a matching rectifier diode small enough and able to operate fast enough to capture the electromagnetic wave oscillations. But the potential of high efficiency and low cost kept scientists working on the technology.

"The physics and the scientific concepts have been out there," said Cola. "Now was the perfect time to try some new things and make a device work, thanks to advances in fabrication technology."

Using metallic multiwall carbon nanotubes and nanoscale fabrication techniques, Cola and collaborators Asha Sharma, Virendra Singh and Thomas Bougher constructed devices that utilize the wave nature of light rather than its particle nature. They also used a long series of tests - and more than a thousand devices - to verify measurements of both current and voltage to confirm the existence of rectenna functions that had been predicted theoretically. The devices operated at a range of temperatures from 5 to 77 degrees Celsius.

Fabricating the rectennas begins with growing forests of vertically-aligned carbon nanotubes on a conductive substrate. Using atomic layer chemical vapor deposition, the nanotubes are coated with an aluminum oxide material to insulate them. Finally, physical vapor deposition is used to deposit optically-transparent thin layers of calcium then aluminum metals atop the nanotube forest. The difference of work functions between the nanotubes and the calcium provides a potential of about two electron volts, enough to drive electrons out of the antennas when they are excited by light.

In operation, oscillating waves of light pass through the transparent calcium-aluminum electrode and interact with the nanotubes. The metal-insulator-metal junctions at the nanotube tips serve as rectifiers switching on and off at femtosecond intervals, allowing electrons generated by the antenna to flow one way into the top electrode. Ultra-low capacitance, on the order of a few attofarads, enables the 10-nanometer diameter diode to operate at these exceptional frequencies.

Georgia Tech associate professor Baratunde Cola measures the power produced by converting green laser illumination to electricity using the carbon nanotube optical rectenna. Credit: Rob Felt, Georgia Tech

"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," Cola explained. "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 rectennas fabricated by Cola's group are grown on rigid substrates, but the goal is to grow them on a foil or other material that would produce flexible solar cells or photodetectors.

Cola sees the rectennas built so far as simple proof of principle. He has ideas for how to improve the efficiency by changing the materials, opening the carbon nanotubes to allow multiple conduction channels, and reducing resistance in the structures.

"We think we can reduce the resistance by several orders of magnitude just by improving the fabrication of our device structures," he said. "Based on what others have done and what the theory is showing us, I believe that these devices could get to greater than 40 percent efficiency."

Explore further: New patented fabrication technique key to new solar power technology

More information: Asha Sharma, Virendra Singh, Thomas L. Bougher and Baratunde A. Cola, "A carbon nanotube optical rectenna," Nature Nanotechnology, 2015.

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4.6 / 5 (5) Sep 28, 2015
This looks to be an entirely new technology. In theory it could be a lot more efficient than solar cells. The ability to generate electricity directly from IR could be huge.
not rated yet Sep 28, 2015
but IR = heat so that's been done.
5 / 5 (3) Sep 28, 2015
"but IR = heat so that's been done."

True but the efficiency stinks when you are talking about waste heat. You need a very high difference in temperatures to produce any meaningful power.
5 / 5 (6) Sep 28, 2015
Wow... diodes operating at petahertz frequencies? It would be great if this inspired some sort of breakthrough for optical computing
not rated yet Sep 28, 2015
Why the carbon nanotube, cats whisker, design?
It will make it hard to improve performance beyond the 1% level, let alone approach 40%.

Previous rectennas have been produced lithographically and though they have not yet reached optical wavelengths their performance was better.
5 / 5 (1) Sep 28, 2015
I agree with MR166, "this could be huge". Of course they need to improve efficiency but then it took Edison a lot of experimentation to produce an efficient light bulb.
not rated yet Sep 29, 2015
I could see it taking a good 10 years plus for this to become a viable technology but imagine the benefits of being able to utilize almost all of the energy put into a system.
3 / 5 (2) Oct 03, 2015
This technology has HUGE potential. Theoretical Photovoltaic efficiencies up near or above 90% with no clear technical barrier, and no toxic compounds involved and very little actual materials of any sort. Some carbon, some salt and a bit of aluminum. Figure a 70% solar --> electricity efficiency transparent flexible film which could easily be applied to any exterior surface, offering 1 kw electric generation for each 3 sq meters of roof, or 4 sq meters of sun-facing exterior wall at an installation cost of perhaps $100 / sq meter, $300 to $400 / kw.

Those efficiency levels have already been demonstrated for wavelengths about 10x that of visible light, it has been just matter of figuring out the miniaturization required. I've been calling for research on nanotubes as antennae for over ten years now.

This research project should receive absolute TOP PRIORITY. Funds, researchers, fabrication facilities, whatever it needs. Nothing else is anywhere near as important.
5 / 5 (1) Oct 03, 2015
Even though this and electric auto batteries on bi-directional PHEV autos WILL put the fossil fuel industry out of business, lets NOT let them hide this one away the way Shell and others have hidden away the Nimh battery, urban rail transit, and dozens of others. WE MUST STAY AWARE!
5 / 5 (1) Oct 08, 2015
This breakthrough could help to optimize the rectennas:
IBM breakthrough has reduced contact resistance:

IBM researchers had to forego traditional contact schemes and invented a metallurgical process akin to microscopic welding that chemically binds the metal atoms to the carbon atoms at the ends of nanotubes. This 'end-bonded contact scheme' allows the contacts to be shrunken down to below 10 nanometers without deteriorating performance of the carbon nanotube devices.

"For any advanced transistor technology, the increase in contact resistance due to the decrease in the size of transistors becomes a major performance bottleneck," Gil added. "Our novel approach is to make the contact from the end of the carbon nanotube, which we show does not degrade device performance. This brings us a step closer to the goal of a carbon nanotube technology within the decade."
Oct 08, 2015
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