However, a newly developed cloaking material that renders objects or people practically invisible could make hiding from infrared detectors much easier.
“What we have shown is an ultrathin stealth ‘sheet.’ Right now, what people have is much heavier metal armor or thermal blankets,” says Hongrui Jiang, professor of electrical and computer engineering at the University of Wisconsin–Madison.
Warm objects—such as human bodies or tank engines—emit heat in the form of infrared light. The new stealth sheet offers significant improvements over other heat-masking technologies.
“It’s a matter of the weight, the cost and ease of use,” says Jiang.
The stealth sheet is less than one millimeter thick and is capable of absorbing approximately 94 percent of the infrared light it encounters. Trapping so much of the infrared light means that warm objects beneath the cloaking material become almost completely invisible to infrared detectors.
The most important aspect of this stealth material is that it strongly absorbs light in the so-called mid- and long-wavelength infrared range—specifically, the type of light emitted by objects that have an approximately human body temperature.
To trap the infrared light, the research team used a unique material called black silicon. Commonly incorporated into solar cells, black silicon absorbs light because the material consists of millions of microscopic needles called nanowires that all point upward like a densely-packed forest. Incoming light becomes trapped and reflects back and forth between the vertical spires, bouncing around within the structure of the material rather than escaping.
Black silicon has long been known to absorb visible light, but Jiang’s team was the first to see the material’s potential for trapping infrared. They then boosted its absorptive properties by tweaking the method they used to create their material.
“We didn’t completely reinvent the whole process, but we did extend the process to much taller nanowires,” explains Jiang, who developed the material in National Science Foundation-supported facilities at UW–Madison.
Creating those nanowires required 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 includes a flexible backing which is interspersed with small air channels. Those air channels prevent the stealth sheet from heating up too quickly as it absorbs infrared light.
But the stealth sheet doesn’t just have applications for hiding something. The research team also demonstrated using the sheet as a high-tech disguise capable of tricking infrared cameras by incorporating electronic heating elements into the stealth sheet.
“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.”
Jiang and colleagues are working to scale up their prototype for real-world applications with assistance from UW–Madison’s Discovery to Product program. They received a U.S. patent in the fall for the material's use in stealth. The Wisconsin Alumni Research Foundation supported the research through its Robert Draper Technology Innovation Fund, and is actively pursuing two additional patent applications. Their full report is described this week in the research journal Advanced Engineering Materials.
For other invisibility technology, check out Researchers Heat Up Invisibility Capabilities.