The devices, called optical rectennas, combine micron-scale antennas (which absorb radiation) and sub-micron diodes (which convert that energy into DC currents) to provide a promising way to efficiently harvest low-grade waste heat. Too small to see with the naked eye, their devices, say the scientists, are roughly 100 times more efficient than similar tools used for energy harvesting.
They achieve that feat through a process called " resonant tunneling ," in which electrons pass through solid matter without spending any energy. Such rectifying antennas - or rectennas - work similarly to radio antennas, but instead of picking up radio waves and turning them into sound, optical rectennas absorb light and heat and convert it into power.
"It's like a radio receiver that picks up light in the form of electromagnetic waves," says Garret Moddel, professor of the Department of Electrical, Computer and Energy Engineering (ECEE) and coauthor of a paper on the study.
Such rectennas could, say the scientists, theoretically, harvest the heat coming from factory smokestacks or bakery ovens that would otherwise go to waste. Some scientists have even proposed mounting these devices on airships that would fly high above the planet's surface to capture the energy radiating from Earth to outer space.
Up to now, rectennas haven't been able to reach the efficiencies needed to meet such goals. In the new study, however, the researchers say they have have designed the first-ever rectennas that are capable of generating power.
"We demonstrate for the first time electrons undergoing resonant tunneling in an energy-harvesting optical rectenna," says Amina Belkadi, lead author of the paper on the research, who recently earned her Ph.D. from ECEE. "Until now, it was only a theoretical possibility."
Study coauthor Garret Moddel adds, "This innovation makes a significant step toward making rectennas more practical. Right now, the efficiency is really low, but it's going to increase."
However, in order to capture thermal radiation and not just