Wednesday, August 08, 2018

"Looking Cool" with an Infrared Invisibility Cloak

Who among us hasn’t dreamed of owning an invisibility cloak? Now, a team at the University of Wisconsin-Madison thinks they have one figured out. There's a catch, though: It only works for infrared light. Although infrared is already invisible to human eyes, it’s extremely useful in security and defense, making this an exciting development.

All objects possess some amount of thermal energy, a property of the particles they’re made of. As these particles shuffle around and vibrate because of their momentum, they release energy in the form of a steady, distinctive glow. Though you may not know it, you’re already deeply familiar with this phenomenon: it’s the reason the coals of a campfire glow red, incandescent bulbs light your home, and the sun bathes the Earth in light.

Not all objects glow the same, however. Take a look at the graph below, which indicates the intensity of each wavelength that makes up the overall thermal—or "blackbody"—radiation of several objects. You’ll notice that they all share the same distinctive curve, but each object’s emission peaks at a different wavelength. In fact, as Wilhelm Wein discovered in 1893, the peak wavelength of any blackbody curve is inversely proportional to the absolute temperature of the emitting object—so hotter objects glow "bluer". This is how astronomers are able to tell the temperature of a star from its color, and how you know that a white-hot piece of iron is hotter than a red-hot one.

The blue curve represents a thermal body slightly cooler than our Sun (5777 Kelvin), while the red curve is typical of a red dwarf (about 3000 Kelvin).
Image Credit: Darth Kule, Wikimedia Creative Commons.
This brings us to the usefulness of infrared sensors. You’ll notice that all of the objects highlighted above—stars, coals, light bulbs—are very hot, at least relative to us. But even objects at room temperature (and cooler) glow with blackbody radiation. It’s a direct consequence of Wein’s Law that their blackbody radiation peaks at wavelengths so long we can’t see them: the infrared.

That’s why infrared detectors are so popular. Even if there is no visible light to see by, these sensors can detect the long-wavelength radiation emitted by people, animals, and even things like cars. Scientists use them to observe animals at night, and people install them outside of their homes for extra security. But they also come in handy in military settings; for example, drones can use them to find targets even when it’s dark or foggy.

But where a technology can be used offensively, there's always a corresponding effort to defend against that use. The team at the University of Wisconsin-Madison, led by Dr. Hongrui Jiang, understands the need for a thin, flexible, and effective infrared shield to protect military operations from these kinds of attacks. Although infrared shields already exist, they’re quite heavy and unwieldy, making them difficult to transport and use. Instead of using costly metamaterials and graphene, Jiang's lab decided to reevalute an older material: black silicon.

Black silicon was first created in the 1980s, and has recently risen in popularity thanks to its applications in the solar power industry. It’s carefully crafted from pure silicon wafers, which are etched to create microscopic spikes sticking up from the surface. These “needles” can be thought of as a thick forest, or a matted carpet. As infrared light from the shielded object hits this nanostructured surface layer on the inside of the cloak, it gets channeled into a series of refractions, which bend the beam deeper into the material, “trapping” it. Since light doesn’t reflect off of the surface, the material appears black rather than silicon’s usual grey—hence its name.

The thick "forest" of silicon is readily apparent in this highly textured sample.
Image Credit: Sedao, Wikimedia Creative Commons.
Although black silicon’s ability to absorb visible light has long been known in the scientific community, Jiang was the first to see its potential in infrared shielding. He teamed up with Dr. Mohamad J. Moghimi and Dr. Guangyun Lin to investigate its absorption properties at a variety of IR wavelengths. In his view, “It turned out to work very well.”

That may be something of an understatement. Once the researchers had refined their production of black silicon, the end result was effective in blocking 94% of the IR light it encountered! This is enough to set it on par with any existing shield technology.

These heatmaps demonstrate how well the new shield can block infrared radiation.
Image Credit: Hongrui Jiang.
One of the most fundamental advances in this research is Jiang and his collaborators’ technique for producing black silicon, a tweak on an existing method known as a wet chemical etch. Wet chemical etching uses silver nanoparticles as a catalyst to allow the chemical etch to eat away the wafer’s smooth surface, leaving behind tall pillars of silicon. Jiang points out that they didn’t invent this process, but they did refine it in order to create taller nanowires of silicon—which in turn boosts its absorptive properties.

However, when light bounces around inside the black silicon, it imparts energy to the nanowires and silver nanoparticles in the form of heat. If the shield heats up too much, it will end up releasing its own radiation, rendering it completely useless! But radiation is only one of a few ways to disperse heat: a hot coal floating in a vacuum can only lose thermal energy by radiation, but if that same coal is plunged into a river, its heat will be transferred away through conduction and it will quickly drop to the same temperature as the water. Taking advantage of this, the team embedded tiny air channels in the back of the shield to keep the entire system ventilated, allowing the shield to properly block infrared light.

Blocking light is only one part of invisibility, however. Imagine a jeep parked in front of a guardrail. With an infrared camera you would see the jeep and the guardrail extending on either side of it. But if you covered the jeep with an IR shield, suddenly you’d see just the guardrail—with a giant gap in the middle! Obviously, that looks suspicious.

Jiang and his colleagues realized this would be a problem, so they embedded arrays of resistors in the back of the sheet. When a current is passed through the resistors they heat up, releasing their own infrared radiation. Jiang explains, “We can turn on some of the resistors to generate a ‘false’, different heat signature than that of the original object, to fool the IR camera.” As if that weren’t exciting enough, he adds, “It can be programmed on the fly.”

Despite its many components, this new “cloak” is just one millimeter thick. It’s also flexible, allowing it to bend around just about any object. As Jiang explains, “Existing technologies use metal sheets or linings or thermal blankets, which are heavy, high cost, and have many drawbacks.” This new shield, on the other hand, is constructed of silicon and polymers, which are cheaper and easier to work with than the graphene and metamaterials required by existing technology. Although it’s still in the experimental phase, Jiang estimates that the new creation could be mass produced in five years.

The invention’s flexibility and light weight means it really is much closer to being a cloak than the traditional bulky shields. It holds the promise of allowing military operations to run undetected where otherwise it would be impossible; we may not be at true Harry Potter-level invisibility cloaks just yet, but this development brings us one step closer.

Eleanor Hook

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