A new energy-harvesting device developed by engineers at the University of Colorado takes advantage of the ghost-like property of electrons to turn excess heat into usable electricity.

The so-called "optical rectennas" utilize a phenomenon known as resonant tunneling, whereby electrons pass -- or tunnel -- through solid matter without expending any energy.

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

Scientists have previously proposed placing rectennas, or rectifying antennas, which can absorb and convert light and heat into energy, on factory smokestacks or even low Earth orbit to capture excess heat.

Previous optical rectenna prototypes have failed to achieve efficiencies that would make such applications feasible.

Now, however, scientists have developed a rectenna capable of converting ambient energy into power. Researchers described their new device in a new paper, published Tuesday in the journal Nature Communications.

"We demonstrate for the first time electrons undergoing resonant tunneling in an energy-harvesting optical rectenna," Belkadi said. "Until now, it was only a theoretical possibility."

The device's efficiency is quite low, but researchers are confident that their design can be perfected over time.

Surprisingly, rectennas have been around for decades, and in their most basic form, they're quite simple -- consisting of only an antenna to absorb radiation and a diode to convert energy into a direct current.

"It's like a radio receiver that picks up light in the form of electromagnetic waves," said co-author Garret Moddel, professor of electrical, computer and energy engineering at Colorado.

The earliest rectennas were designed to capture microwaves, but to capture thermal radiation, rectifying antennas are necessary. Unfortunately, shrinking antennas tends to increase their resistance, limiting their efficiency.

"You need this device to have very low resistance, but it also needs to be really responsive to light," Belkadi said. "Anything you do to make the device better in one way would make the other worse."

Instead of solving this seemingly impossible problem, researchers attempted to avoid the tradeoff altogether by taking advantage of the quantum phenomenon known as resonant tunneling.

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 their potential efficiency.

In the new microscopic device, researchers added a second insulator to their antennas, creating what's called a "quantum well." At certain energy levels, electrons can tunnel through the well without expending any 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 said. "They just go zooming through."

To test their new technology, researchers arranged an array of 250,000 bowtie-shaped rectennas on a hot plate in the lab. The rectennas captured less than 1 percent of the hot plate's thermal energy, but researchers estimate it's only a matter of time before that number increases.

"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."

Eventually, scientists 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 said.

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