‘Stealth sheet’ hides hot objects from infrared detectors

28 Jun 2018

Hot bodies, whether human or military vehicles, are hidden by sheet that offers significant improvements over other masks.

Infrared cameras help drones find their targets, even in the dead of night or through smoke and fog. Hiding from such detectors could become much easier, thanks to a new cloaking material that renders objects and people practically invisible, say its developers.

“What we have shown is an ultrathin stealth sheet. Right now, what people use tends to be much heavier metal armor or thermal blankets,” said Hongrui Jiang, the Lynn H. Matthias Professor and Vilas Distinguished Achievement Professor of Electrical and Computer Engineering at the University of Wisconsin-Madison.

Warm objects like human bodies or tank engines emit heat as infrared light, and the new stealth sheet, described in Advanced Engineering Materials, offers substantial improvements over other heat-masking technologies, says the Wisconsin team. “It’s a matter of the weight, the cost and ease of use,” commented Jiang.

Measuring less than one millimeter wide, the thin sheet absorbs approximately 94 percent of the infrared light it encounters. Trapping so much light means that warm objects beneath the cloaking material become almost completely invisible to infrared detectors.

Strong absorption in MW- and LWIR

Importantly, the stealth material can strongly absorb light in the so-called mid- and long-wavelength infrared range, which is the type of light emitted by objects at approximately human body temperature. Furthermore, by incorporating electronic heating elements into the stealth sheet, the researchers created a high-tech disguise for tricking infrared cameras.

“You can intentionally deceive an infrared detector by presenting a false heat signature,” says Jiang. “It could conceal a tank by presenting what looks like a simple highway guardrail.”

To trap infrared light, Jiang and colleagues turned to a special material called black silicon, which is commonly incorporated into solar cells. Black silicon absorbs light because it consists of millions of microscopic nanowires, pointing upward like a densely-packed forest—incoming light reflects back and forth between the vertical spires, bouncing around within the material instead of escaping.

Although black silicon has long been known to absorb visible light, Jiang and colleagues were the first to see the material’s potential for trapping infrared, and they boosted its absorptive properties by tweaking the method through which they created their material.

“We didn’t completely reinvent this whole process, but we did extend the process to much taller nanowires,” said Jiang.

Prototype under development

They developed nanowires by using tiny particles of silver to help etch down into a thin layer of solid silicon, which results in a thicket of tall needles. Both the nanowires and the silver particles contribute to absorbing infrared light.

The researchers’ black silicon also has a flexible backing interspersed with small air channels. Those air channels prevent the stealth sheet from heating up too quickly as it absorbs infrared light.

Currently, Jiang and colleagues are working to scale up their prototype for real-world applications with assistance from the UW-Madison Discovery to Product (D2P) program. They received a US patent in fall 2017 for the material’s use in stealth, and the Wisconsin Alumni Research Foundation, or WARF—UW-Madison’s patenting and technology licensing arm—is actively pursuing two additional patent applications.

WARF supported the research through its Robert Draper Technology Innovation Fund. The team also used National Science Foundation-supported shared facilities at UW-Madison.