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Stealth Sheet Conceals Hot Objects from IR

Photonics Spectra
Sep 2018
An ultrathin cloaking material has been developed that can hide hot objects, such as human bodies and military vehicles, from IR cameras. The new “stealth sheet” is <1 m thick and absorbs approximately 94 percent of the IR light it encounters, making hot objects beneath the cloaking material almost completely invisible to IR detectors.

To make the stealth material capable of absorbing light in the MIR and LWIR range — the wavelengths emitted by objects that are the approximate temperature of a human body — researchers from the University of Wisconsin-Madison integrated and transferred metallic-dielectric nanostructures and microscale IR emitters onto thin, flexible substrates. The nanostructures can absorb and scatter a broad band of IR wavelengths, reducing reflection and transmission to below 5 percent across a range from 2.5 to 15.5 μm, and thus reducing significantly the amount of IR signals propagating toward detectors.

Cloaking material hides hot objects from IR, University of Wisconsin-Madison.

A stealth sheet can hide hot objects like human bodies or military vehicles from IR cameras. Courtesy of Hongrui Jiang.

Microemitters in the material, which are thermally isolated from the broadband absorbers, can present false thermography to deceive IR detectors and heat-sensing cameras.

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

To trap IR light, researchers used black silicon, a material known to absorb visible light. They boosted the absorptive properties of black silicon by making a few tweaks to the way the material is typically created.

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

Hongrui Jiang, professor of electrical and computer engineering at the University of Wisconsin-Madison.
Hongrui Jiang, professor of electrical and computer engineering at the University of Wisconsin-Madison. Courtesy of UW-Madison College of Engineering.

Silver nanoparticles were used to etch nanowires into a thin layer of silicon, creating a thicket of tall nanowires. Incoming light reflects back and forth between the millions of nanowires in the black silicon, bouncing around within the material instead of escaping. 

To prevent the stealth sheet from heating up too quickly as it absorbs light, the researchers added a flexible backing interspersed with small air channels to the black silicon.

Results of the research show that the nanostructures used in the stealth sheet can almost completely conceal the thermal emission from objects and blend them into their surroundings. Researchers are working to scale up their prototype for real-world applications. They received a U.S. patent for the material’s use in stealth.

The research was published in Advanced Engineering Materials (doi:10.1002/adem.201800038). 

GLOSSARY
thermal imaging
The process of producing a visible two-dimensional image of a scene that is dependent on differences in thermal or infrared radiation from the scene reaching the aperture of the imaging device.
broadband
Indicating a capability to deal with a relatively wide spectral bandwidth.
Research & TechnologyeducationAmericasimaginglight sourcesmaterialsSensors & Detectorscamerasinfrared camerasthermal imagingdefenseindustrialinfraredbroadbandnanonanostructuresilicon nanowiressilver nanoparticlesblack siliconsilicon photonicsTech Pulse

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