New Coating Enhances Thermal Imaging

You’ve likely seen it in a popular espionage film. To fully understand the threat within a building, the hero uses an infrared camera to detect the heat signatures of the people waiting inside. In the real world, however, thermal imaging isn’t quite so precise. For example, when temperatures heat up windows, the application’s ability to image objects on the other side is obscured.

Now, researchers at Rice University have developed a unique coating leveraging nanoscale resonators. The team, led by Gururaj Naik, an associate professor of electrical and computer engineering, is able to suppress the thermal radiation from hot windows toward the camera, allowing a clear image. The team’s investigative article, “Thermal imaging through hot emissive windows” recently appeared in Nature. 

Persistent problem

Naik said he had been talking about lossy systems, or quantum systems where energy is lost due to interactions with the environment, with Henry Everitt, a senior scientist at the United States Army Research Laboratory. 

“We started talking about thermal imaging because anything that has thermal light coming out of it must have some losses,” Naik said. “[Everitt] said this is a problem that the Army has been having for a long time. Hot windows can inhibit the ability to do thermal imaging as they do remote monitoring, but it also affects your ability to monitor a chemical reaction happening in a hot chamber because most of the emission you’ll capture will be from the hot window, not the contents of the reactor.”

As the two talked more, they decided to try to address the issues, as well as the longstanding “Narcissus” effect, where even heat coming off a camera itself can interfere with imaging. Naik said it inspired the research team to delve into “many different aspects of engineering and fundamental science, including optics and information science” to come up with a workable solution.

New thinking

The group started by testing thin films, optical, and anti-reflective coatings, but realized they were not sufficient. But while this approach did not immediately yield a solution, it did, Naik said, help the team understand the underlying physics and realize they would be best guided by a non-mission quantum mechanical approach.

“We realized we needed to create something that doesn’t simply reduce reflection passively but also exploits resonances,” he said. “We started thinking about it as an information science problem. We want to see something behind the window. The window adds its own thermal emission, which acts as noise. We needed to enhance the signal-to-noise ratio better so we could capture the signal we wanted.”

With that understanding in place, Naik and colleagues developed a unique coating using silicon nanoscale resonators. When the resonators are organized in the right way, they could suppress the thermal emission from the window toward the camera without diminishing the transmission of the object they wanted to image on the other side.

“We used plasma-enhanced chemical vapor deposition (PECVD) of silicon, which eventually becomes polycrystalline silicon. The nanostructuring helped release thermal stress because the structures were so small,” he explained. “We then made many small tweaks to the array so that it worked at temperatures even as high as 600 degrees Centigrade—and we believe we can eventually take this device up to 1200 degrees Centigrade.”

The future

Naik said the team is very excited by this breakthrough advance and will continue working to refine it. He said similar non-mission quantum mechanic frameworks can also bring about advances in fields including security, surveillance, industrial research, energy harvesting, sensing, and even medical diagnostics. But first, he and his colleagues are working to improve their current coating.

“The first thing we need to do is to make [the coating] applicable to any general condition,” Naik said. “Then, can we make it easier to apply? Can we apply it as a coating rather than relying on a planar fabrication process? We could even, perhaps, come up with a bottom-up synthesis approach using nanoparticle growth and self-assembly so we could one day just roll the coating on like paint.”

Naik admitted there is still a lot of work to do. But he believes the most important result of this work is showing that engineers can exploit lossy systems using a non-mission quantum mechanical framework to create new functionality. It all starts, he said, by thinking about ways to maintain transparency while reducing noise.

“We have thought of these thermal losses as an undesirable material property for a very long time,” he said. “But we can use them in new ways. And it can have an extremely big impact, influencing how we design devices in the future. We can use these thermal losses as a new axis in design, which adds a new space of functionality that was not possible before.” 

Kayt Sukel is a technology writer and author in Houston.