First optical rectenna converts light to direct current

The first optical rectenna, a device that combines the functions of an antenna and a rectifier diode, can convert light directly into direct current.

The new device, developed by researchers at the Georgia Institute of Technology, 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 solar energy.

“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,” 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 1% efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture.”

In the new optical rectennas, 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 1%.

The researchers hope to boost that output through optimisation techniques and believe that a rectenna with commercial potential may be available within a year.

Developed in the 1960s and 1970s, rectennas have operated at wavelengths as short as 10 microns, but for more than 40 years researchers have been attempting to make devices at optical wavelengths. 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 utilise the wave nature of light rather than its particle nature. The devices operated at a range of temperatures from 5 to 77°C.

Fabricating the rectennas begins with growing forests of vertically aligned carbon nanotubes on a conductive substrate. Using atomic layer chemical vapour deposition, the nanotubes are coated with an aluminium oxide material to insulate them. Finally, physical vapour deposition is used to deposit optically transparent thin layers of calcium then aluminium 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 carbon nanotube antennas when they are excited by light.

In operation, oscillating waves of light pass through the transparent calcium-aluminium 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. Ultralow capacitance, on the order of a few attofarads, enables the 10-nanometre diameter diode to operate at these exceptional frequencies.

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.

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), was reported the journal Nature Nanotechnology.

Image caption: 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.