Scientists develop coating for enhanced thermal imaging through hot windows

"Say you want to use thermal imaging to monitor chemical reactions in a high-temperature reactor chamber," said Gururaj Naik, an associate professor of electrical and computer engineering at Rice and corresponding author on the study. "The problem you'd be facing is that the thermal radiation emitted by the window itself overwhelms the camera, obscuring the view of objects on the other side."

A possible solution could involve coating the window in a material that suppresses thermal light emission toward the camera, but this would also render the window opaque. To get around this issue, the researchers developed a coating that relies on an engineered asymmetry to filter out the thermal noise of a hot window, doubling the contrast of thermal imaging compared to conventional methods.

The core of this breakthrough lies in the design of nanoscale resonators, which function like miniature tuning forks trapping and enhancing electromagnetic waves within specific frequencies. The resonators are made from silicon and organized in a precise array that allows fine control over how the window emits and transmits thermal radiation.

"The intriguing question for us was whether it would be possible to suppress the window's thermal emission toward the camera while maintaining good transmission from the side of the object to be visualized," Naik said. "Information theory dictates a 'no' for an answer in any passive system. However, there is a loophole -- in actuality, the camera operates in a finite bandwidth. We took advantage of this loophole and created a coating that suppresses thermal emission from the window toward the camera in a broad band but only diminishes transmission from the imaged object in a narrow band."

This was achieved by designing a metamaterial comprised of two layers of different types of resonators separated by a spacer layer. The design allows the coating to suppress thermal emissions directed toward the camera while remaining transparent enough to capture thermal radiation from objects behind the window.

"Our solution to the problem takes inspiration from quantum mechanics and non-Hermitian optics," said Ciril Samuel Prasad, a Rice doctoral engineering alum and first author on the study.

The result is a revolutionary asymmetric metawindow capable of clear thermal imaging at temperatures as high as 873 K (approximately 600 C).

The implications of this breakthrough are significant. One immediate application is in chemical processing, where monitoring reactions inside high-temperature chambers is critical. Beyond industrial uses, this approach may revolutionize hyperspectral thermal imaging by addressing the long-standing "Narcissus effect," where thermal emissions from the camera itself interfere with imaging. The researchers envision applications in energy conservation, radiative cooling and even defense systems, where accurate thermal imaging is essential.

"This is a disruptive innovation," the researchers noted. "We've not only solved a long-standing problem but opened new doors for imaging in extreme conditions. The use of metasurfaces and resonators as design tools will likely transform many fields beyond thermal imaging from energy harvesting to advanced sensing technologies."

Henry Everitt, senior scientist at the United States Army Research Laboratory and adjunct faculty at Rice, is also an author on the study.

The research was supported by the United States Army Research Office under cooperative agreement number W911NF2120031.