Rectifying antennas – "rectennas" – are used as parasitic power capture devices that absorb radio frequency (RF) energy and convert it into usable electrical power. Constructing such devices to absorb and rectify at optical wavelengths has proved impractical in the past, but the advent of carbon nanotubes and advances in microscopic manufacturing technology have allowed engineers at the Georgia Institute of Technology to create rectennas that capture and convert light to direct electrical current. The researchers believe that their creation may eventually help double the efficiency of solar energy harvesting.

First created more than 40 years ago, rectennas have been used to capture and convert energy at various wavelengths as short as ten microns (in the infrared range), and have found use in such things as re-using localized RF energy to power near-field communications (NFC). However, researchers have been trying to create devices that operate at visible wavelengths with little success.

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

These advances enabled researchers to cultivate an array of billions of vertically-aligned carbon nanotubes on top of a silicon substrate using chemical vapor deposition. Each of these nanotubes is glazed with an aluminum oxide insulator, and the whole array is capped off with optically-transparent thin layers of calcium then aluminum to act as an anode.

The multiwall carbon nanotubes harness light energy by oscillating in response to the incoming optical radiation. This generates a charge that is then passed through inbuilt rectifiers, which convert alternating current into direct current. These rectifiers switch on and off in response to the oscillations at incredible speeds in the petahertz (one quadrillion times per second) range and "push" electrons across the uppermost electrode to create a small amount of direct current.

Despite the fact that these devices currently operate at less than one percent efficiency, the Georgia Tech engineers are working on optimization techniques they believe could greatly improve their performance and lead to practical application in solar technology.

"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 Cola. "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 team outlines the research in the short video below.

The results of this work were recently published in the journal Nature Nanotechnology.

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