‘Ghostly’ property of electrons enables efficient energy harvesting in device

These devices, which are too small to see with the naked eye, are roughly 100 times more efficient than similar tools used for energy harvesting, the researchers said. The “optical rectennas” utilise a process known as resonant tunnelling, whereby electrons pass – or tunnel – through solid matter without expending any energy.

“They go in like ghosts,” said lead author Amina Belkadi, who recently earned her doctorate in electrical, computer and energy engineering at Colorado.

Scientists have previously proposed placing rectennas, short for rectifying antennas, which can absorb and convert light and heat into energy, on factory smokestacks or even in low Earth orbit to capture excess heat. But, so far, rectennas haven’t been able to reach the efficiencies needed to meet those goals.

Now, however, scientists have developed a rectenna capable of converting ambient energy into power. “We show for the first time electrons undergoing resonant tunnelling in an energy-harvesting optical rectenna,” Belkadi said. “Until now, it was only a theoretical possibility.”

Garret Moddel, professor of ECEE, said that the study is a major advance for this technology. “This innovation makes a significant step toward making rectennas more practical,” he said. “Right now, the efficiency is really low, but it’s going to increase.”

Rectennas have been around since 1964 when engineer William C Brown used microwaves to power a small helicopter. Researchers have said they’re relatively simple tools, made up of an antenna, which absorbs radiation, and a diode, which converts that energy into DC currents.

But researchers have stressed that to capture thermal radiation and not just microwaves, rectennas need to be tiny – many times thinner than human hair – which can cause a range of problems. The smaller an electrical device is, for example, the higher its resistance becomes, which can shrink the power output of a rectenna.

“You need this device to have very low resistance, but it also needs to respond to light,” Belkadi added. “Anything you do to make the device better in one way would make the other worse.”

In traditional antennas, an insulator is used to capture the energy from electrons as they pass through. But insulators increase an antenna’s resistance, limiting its potential efficiency.

In the new microscopic device, researchers added a second insulator to their antennas, creating what’s known as a “quantum well.” At certain energy levels, electrons can tunnel through the well expending no energy.

“If you choose your materials right and get them at the right thickness, then it creates this sort of energy level where electrons see no resistance,” Belkadi explained. “They just go zooming through.”

To test their new technology, the researchers arranged an array of 250,000 bowtie-shaped rectennas on a hot plate in the lab. The rectennas captured less than one per cent of the hot plate’s thermal energy, but researchers estimate that figure will increase.

“If we use different materials or change our insulators, then we may be able to make that well deeper,” Belkadi said. “The deeper the well is, the more electrons can pass all the way through.”

The researchers predict the technology will be used to capture excess heat emanating from solar panels or to harness all that thermal energy escaping into space. “If you can capture heat radiating into deep space, then you can get power anytime, anywhere,” Moddel concluded.