Jeff Glorfeld
Jeff Glorfeld is a former senior editor of The Age newspaper in Australia, and is now a freelance journalist based in California, US.
The research team that demonstrated the first optical rectenna, in 2015, is now reporting a two-fold efficiency improvement in the devices, which could lead to a new way of efficiently capturing solar energy, and perhaps one day even help power a spacecraft to Mars.
In September 2015, in the journal Nature Nanotechnology, US researchers Baratunde Cola and Thomas Bougher, working at the Georgia Institute of Technology, reported the first optical rectenna – a device that had been proposed theoretically for more than 40 years, but never demonstrated.
Although Nicola Tesla introduced and demonstrated the concept of wireless power more than 100 years ago, it was American inventor William C. Brown, working for US electronics company Raytheon, who was the key developer of the rectenna – a combination of rectifier and antenna – in the 1960s.
What Cola and Bougher did was to make the devices work at optical wavelengths.
A rectenna harvests ambient wireless electromagnetic energy through an antenna attached to rectifying diodes, which convert the received energy into DC power. The two key advantages they have over batteries is an almost unlimited lifespan, and they are non-polluting. There’s nothing to throw away.
Now, as reported in the journal Advanced Electronic Materials, the researchers have taken “a significant leap forward in both fundamental understanding and practical efficiency for the optical rectenna device”.
The improvements could allow the optical rectennas – which convert electromagnetic fields at optical frequencies directly to electrical current – to operate low-power devices such as temperature sensors.
Ultimately the researchers believe their device design – a combination of a carbon nanotube antenna and diode rectifier – could compete with conventional photovoltaic technologies for producing electricity from sunlight and other sources. The same technology used in the rectennas could also directly convert thermal energy to electricity.
“It opens up this technology to many more researchers who can join forces with us to advance the optical rectenna technology to help power a range of applications, including space flight,” Cola says.
The research has been supported by the US Army Research Office, and by the National Science Foundation.
Carbon nanotubes conduct heat and electricity far better than copper. They have a diameter measured on the nanometer scale. (A nanometer is one-billionth of a meter, or about 10,000 times smaller than a human hair.)
Optical rectennas operate by coupling light’s electromagnetic field to an antenna, in this case an array of carbon nanotubes whose ends have been opened. The electromagnetic field creates an oscillation in the antenna, producing an alternating flow of electrons. When the flow reaches a peak at one end of the antenna, the diode closes, trapping the electrons, then re-opens to capture the next oscillation, creating a current flow.
The switching must occur at terahertz frequencies to match the light. The junction between the antenna and diode must provide minimal resistance to electrons flowing through it while open, yet prevent leakage while closed.
“The name of the game is maximising the number of electrons that get excited in the carbon nanotube, and then having a switch that is fast enough to capture them at their peak,” Cola explains. “The faster you switch, the more electrons you can catch on one side of the oscillation.”
The researchers initially used calcium as the metal in their diode junction, but it breaks down rapidly in air, meaning the device had to be encapsulated during operation, which made it impractical for most applications and difficult to fabricate.
So Cola and his team switched to aluminium, along with a material composed of alumina (Al2O3) and hafnium dioxide (HfO2). The combination coating for the carbon nanotube junction, created through an atomic deposition process, provides the quantum mechanical electron tunneling properties required by engineering the oxide electronic properties instead of the metals, which allows air-stable metals with higher work functions than calcium to be used.
Rectennas fabricated with the new combination have remained functional for as long as a year. Other metal oxides could also be used, Cola says.
The early version reported in the journal produced power at microvolt levels. The rectenna now produces power in the millivolt range and conversion efficiency has gone from 10-5 to 10-3 – still very low, but a notable gain.
“Though there still is room for significant improvement, this puts the voltage in the range where you could see optical rectennas operating low-power sensors,” Cola says.
“There are a lot of device geometry steps you could take to do something useful with the optical rectenna today in voltage-driven devices that don’t require significant current.”
Cola believes the rectennas could be useful for powering internet-of-things devices, especially if they can be used to produce electricity from scavenged thermal energy. For converting heat to electricity, the principle is the same as for light – capturing oscillations in a field with the broadband carbon nanotube antenna.
“People have been excited about thermoelectric generators, but there are many limitations on getting a system that works effectively,” he says. “We believe that the rectenna technology will be the best approach for harvesting heat economically.”
In future work, the research team hopes to optimise the antenna operation, and improve their theoretical understanding of how the rectenna works. One day, Cola hopes the devices will help accelerate space travel, producing power for electric thrusters that will boost spacecraft.
“Our end game is to see carbon nanotube optical rectennas working on Mars, and in the spacecraft that takes us to Mars,” he says.
