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Optical Rectennas Show Aptitude in Waste Heat Capture

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BOULDER, Colo., May 27, 2021 — 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).

Published: May 2021
Glossary
quantum
The term quantum refers to the fundamental unit or discrete amount of a physical quantity involved in interactions at the atomic and subatomic scales. It originates from quantum theory, a branch of physics that emerged in the early 20th century to explain phenomena observed on very small scales, where classical physics fails to provide accurate explanations. In the context of quantum theory, several key concepts are associated with the term quantum: Quantum mechanics: This is the branch of...
quantum well
A quantum well is a structure in quantum mechanics that confines particles, such as electrons or holes, in one spatial dimension. This confinement leads to quantized energy levels, creating a potential well in which the particles are restricted to move. In semiconductor physics and device engineering, quantum wells are commonly used to create electronic or optical devices. These structures are typically thin layers (often on the order of nanometers) sandwiched between layers of a different...
Research & TechnologyOpticsenergyquantumquantum wellheatoptical rectennathermalthermal energythermal energy captureUniversity of ColoradoUniversity of Colorado at BoulderUniversity of Colorado BoulderUC BoulderGarret ModdelAmina BelkadiNature Communicationselectricityclean energygreen energy

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