Rectennas act as very-high-frequency radio receivers. The devices, made mainly of a diode and an antenna, have been used to support applications like infrared (IR) sensing and wireless communications.

Now scientists from CU Boulder want to expand the capability of rectennas by creating optical devices that harvest power from wasted heat.

The optical rectennas, which are too small to see with the naked eye, are roughly 100 times more efficient than similar tools used for energy harvesting, according to its inventors.

The energy harvesting is achieved through a mysterious, perhaps even spooky process called “resonant tunneling”— in which electrons pass through solid matter without spending any energy.

“They go in like ghosts,” said lead author Amina Belkadi , who recently earned her PhD from CU Boulder's Department of Electrical, Computer and Energy Engineering (ECEE).

Belkadi and her team's conclusions were presented in a paper published this month in the journal Nature Communications .

The demonstration — half a nanowatt of power from the heat provided by a hot plate — is a small sample of potentially game-changing applications in renewable energy, according to the CU Boulder team.

Working rectennas, for example, could, theoretically, harvest the wasted heat coming from factory smokestacks or bakery ovens. Some scientists have even proposed mounting rectennas on high-flying aircraft that capture the energy radiating from Earth to outer space.

How Do Rectennas Work?

The rectifying antennas, or rectennas, act like car radio antennas, says Belkadi; instead of picking up radio waves, however, the devices absorb light and convert it into power.

In a traditional rectenna, electrons must pass through an insulator in order to generate power. The insulators add resistance, reducing the amount of electricity that engineers can gather.

The CU Boulder team had a counter-intuitive idea: What if adding an insulator could somehow offer more power?

An added insulator, it turns out, created an energetic phenomenon called a quantum "well."

The researchers demonstrated that when the electrons hit the well with just the right energy, they can tunnel through the insulators...ghost-like.

“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.”

And the faster you can zoom, the greater amount of power you can harvest.

To test the spooky effect, Belkadi and her colleagues arrayed a network of about 250,000 bowtie-shaped rectennas onto a hot plate in the lab. The devices were able to capture less than 1% of the heat produced.

The demonstrated energy efficiency is low, demonstrating 0.5 nW/m2 of power, but the goal is to reach 1 mW/m2.

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

In a short interview with Tech Briefs below, Belkadi explains the plan to increase energy efficiency and envisions new ideas for energy harvesting, like solar cells everywhere.

Tech Briefs: What does your antenna look like?

Amina Belkadi: The main components here are the antenna and the high-speed diode. Our group specializes in making those fast diodes.

The diode is at the feedpoint of the bowtie antenna and then there are leads to measure signals.

Because we're looking at heat as our source, the frequencies we're dealing with are in terahertz. That means our antenna and diode sizes need to be very small, and so we're dealing with quantum physics rather than regular physics laws and quantum mechanics rules are harder to follow and tweak. Things are not that simple in quantum.

The components of the CU Boulder-developed rectenna. (Image Credit: Belkadi)

Tech Briefs: What other design challenges do you have, given the small sizes of the components?

Amina Belkadi: Another size challenge comes from fabrication. We need extremely thin insulators in our diodes: less than 1 nm. Fabrication techniques limit how thin you can go reliably. It takes years to perfect deposition conditions of a single material. For these diodes, the main study approach is usually a material study. So you can imagine how hard it is to switch materials and try to tune them to produce extremely thin layers. We face challenges and questions of: "Is the film continuous when you go that thin? Is it repeatable?"

Tech Briefs: How much energy were you able to capture in your demonstration? How much energy is needed before we can consider this technology in real-world applications, and how possible is that kind of capture?

Amina Belkadi: Our initial proof-of-concept demonstration generated 0.5 nW/m2 or power. That is a very small number compared to what we need to call this technology efficient. The goal would be at least 1 mW/m2. Arriving there would require better diodes (a rectification efficiency of at least 1000x where they are right now) and other improvements to the system such as a compensation structure to improve coupling efficiency. Resonant tunneling has proven it's possible, but there is still significant work ahead.

Tech Briefs: Can you take me through a specific exciting application that you envision with rectennas gathering waste heat and turning it into power?

Amina Belkadi: I'd love to see a rectenna be combined with solar cells everywhere so that solar cells can harvest energy during the day from the sun and rectennas could harvest heat radiated from earth at night. Any source of heat is an ideal spot for a rectenna! When you look at Earth from outer space at night with an infrared camera, you notice how much heat we generate — rectennas could generate energy and reduce the usage of batteries at night.

Tech Briefs: I had a bit of trouble understanding the “Ghost”-like physics. What inspired you to try that kind of mechanism?

Amina Belkadi: When you go down to really small dimensions, quantum physics is at play. You can think of insulators in metal-insulator-metal (MIM) diodes as thick walls. Electrons are trying to go through those walls, and they succeed with a certain probability at specific energy levels. What resonant tunneling does is it allows electrons at a certain energy level to go through those walls with 100% probability. Meaning they don't lose any energy and they go through without resistance — hence, the ghost analogy.

You can think of electrons in the regular scenario going in at multiple energies with different probabilities. Resonant tunneling provides electrons at a specific single energy level with the ability to go in with 100% transmission probability. That is what reduced diode resistance.

Tech Briefs: Where are rectennas being used currently? What will need to happen before we have a mainstream adoption of rectennas for purposes like renewable energy?

Amina Belkadi: Before we can have rectennas as energy harvesting devices, we would need 100x-1000x improvement in diode rectification efficiency. Resonant tunneling brought us 100-1000x closer to the goal but not quite there. Perhaps having two or three resonant tunneling energy levels would do the trick, except that is a very difficult task. It will need significant research in material science to study and understand the behavior of certain thin films at terahertz frequencies. Perhaps some other tunneling mechanism that we haven't thought of yet or perhaps a combination of effects.

Tech Briefs: What will you be working on next?

Amina Belkadi: I'm currently working on a couple of papers from my PhD work: 1) demonstration of waste heat harvesting using an array of MIM-diode rectennas and 2) a compensation structure for terahertz diodes and antennas.

Tech Briefs: How does your technology/approach differ from existing rectenna-energy technologies (like the one shown here from Georgia Tech)?

Amina Belkadi: That's such a good video. Professor Cola has visited our labs and we've had such great discussions about rectenna. For one, we are not dealing with the same frequency range. Our application is waste heat, which peaks at 10.6 um (30 THz). The Georgia Tech rectennas are attempting to harvest incoming light with a wide range of frequencies that peak at 500 nm (When they tested their rectennas in our lab with our lasers and frequencies, their rectennas didn't respond.).

Another main difference is we're using an MIM diode whereas their rectenna uses carbon nanotubes. Our rectenna configuration is older, and the innovation for us relies in engineering that MIM diode to be more efficient. Cola and team presented a rectenna that can capture radiation in the carbon nanotube and rectify it at the tip (the first of its kind). We're very excited about that technology and hope others will pick it up and study it further to see where it could go.

What do you think? Share your questions and comments below.