Researchers from the University of Colorado Boulder have developed optical rectennas (short for “rectifying antennas”) capable of capturing excess heat and turning it into usable electricity. The optical rectennas are too small to see with the naked eye and roughly 100× more efficient than similar tools used for energy harvesting. The devices achieve their high level of efficiency through the process of resonant tunneling, in which electrons pass through solid matter without spending energy. Amina Belkadi, the lead author of a paper describing the research, compared the rectennas to ghosts. They work in a way similar to an antenna found on a radio. Rather than picking up radio signals, optical rectennas absorb light and heat to convert them into power. In theory, rectennas could harvest the heat from factory smokestacks or from a bakery oven’s chimney, energy that would otherwise go to waste. However, rectennas have been unable to meet the efficiencies required to generate power since their introduction in 1964. The problem lies in their design: To capture thermal radiation, rectennas need to be incredibly small — many times smaller than a human hair. However, as an electrical device shrinks, resistance tends to increase, which can shrink power output. “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.” Belkadi and her colleagues developed an approach that relies on a property of the quantum realm to sidestep the bottleneck. In a traditional antenna, she said, electrons must pass through an insulator to generate power. The insulators add resistance, which reduces how much power is generated. In the current study, the researchers added two resonators instead of just one, to create a quantum well. If electrons hit the well with just the right energy, they can use it to tunnel through the two insulators with no resistance. A graduate student in the lab of co-author Garret Moddel, a professor of electrical, computer, and energy engineering, had theorized that such spectral behavior could be possible in optical rectennas, though no one had been able to prove it. “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 the effect, Belkadi and her colleagues arrayed a network of about 250,000 rectennas, which are shaped like tiny bow ties, onto a hot plate and turned on the heat. The devices were able to capture less than 1% of the heat produced by the hot plate, though the team believes the numbers will eventually go up. The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-23182-0).