Optical Rectenna Converts Light Waves into Direct Current

A research team from the Georgia Institute of Technology has developed the first optical rectenna. It combines the functions of an antenna and a rectifier diode to convert light waves into direct current. Based on metallic carbon nanotubes and the rectifiers fabricated onto them, the rectenna is the result of years of research into making the antennas and other components small enough to channel optical wavelengths.

The carbon nanotubes function as antennas to capture light. The light waves create an oscillating charge when they make contact with the nanotube antennas, sending the charge through the rectifier diodes attached to them. The rectifier devices switch on and off at femtosecond intervals, creating a small direct current. In order to produce a significant current, billions of rectennas have to work together in an array.

The efficiency of the optical rectenna remains below one percent, but the impact would be significant if the power output could be increased. With higher efficiencies, the optical rectennas could open the door to photodetectors that would operate without the need for cooling and energy harvesters that would convert waste heat to electricity. It could even result in “solar cells that are twice as efficient at a cost that is ten times lower,” says Baratunde Cola, an associate professor of Mechanical Engineering at Georgia Tech and one of the chief researchers.

Using nanoscale fabrication methods, the research team was able to construct antennas and rectifier diodes small enough to channel optical wavelengths. The rectennas, which operate at temperatures from 5 to 77 degrees Celsius, are fabricated by growing forests of carbon nanotubes on a conductive substrate. They are then insulated with an aluminum-oxide material. After that, thin layers of calcium and aluminum are deposited on top of the nanotube forest.

The difference of work functions between the nanotubes and calcium potentially provide about two electron volts. When excited by light, electrons are driven out of the carbon nanotube antennas. At the same time, the light waves serve to switch the rectifiers, allowing the electrons generated by the antenna to flow one way into the electrode.

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This schematic shows the components of the optical rectenna developed at the Georgia Institute of Technology. (Image courtesy of Thomas Bougher, 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 explains. “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.”

In talking about the new technology, Cola and his fellow researchers—Asha Sharma, Virendra Singh, and Thomas Bougher—stressed that research would continue into increasing the conversion efficiency. Cola stated that changing materials, reducing resistance, and opening the carbon nanotubes to allow multiple conduction channels were avenues for additional research. The final step is to grow the rectennas on foil or other materials that could yield flexible photodetectors.

The research was supported by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center, and the Army Research Office (ARO). The research was published in a recent issue of the journal Nature Nanotechnology.