Invisibility cloaks have been around in various forms since 2006, when the first cloak based on optical metamaterials was demonstrated. The design of cloaking devices has come a long way in the past seven years, as illustrated by a simple, yet highly effective, radar cloak developed by Duke University Professor Yaroslav Urzhumov, that can be made using a hobby-level 3D printer.

As envisioned by Harry Potter and DARPA, invisibility cloaks are an important new direction for camouflage technology. In contrast to conventional stealth technology, which concentrates on reducing the detection signature (radar cross section, heat signatures, optical detection, etc.) of an object, invisibility cloaks work by making it seem as if radar and light flows around the cloaked object. When successfully accomplished, neither the cloaked object nor the cloak will be detected.

How a cloak works can be illustrated by an analogy offered by Duke University Professor David Smith. Imagine a fabric in which the threads are optical fibers. As seen on the left of the image below, light will travel freely from one edge to the opposite edge of a piece of this fabric. If an opaque object is placed so that it blocks some of the light, it is equivalent to cutting a hole in the optical fiber fabric, as seen in the top right-hand image.

In the invisibility cloak at the bottom of the above image, the optical fibers are not cut, but rather bend around the object, with the result that light continues to pass through the fabric without any visible sign that the object exists. Differences in the travel distance for light passing through the various fibers is being ignored, as the resulting phase changes can also be compensated.

Cloaking technology is a happy offspring of the new science of optical metamaterials, in which artificially structured materials display a range of optical properties that cannot be attained using ordinary glasses and crystals. The newest Duke microwave cloak that looks like a disc with oddly-shaped holes dotted throughout is made from only two materials – ABS plastic and air. Because the cloak consists of a single piece of plastic, “essentially anyone who can spend a couple thousand dollars on a non-industry grade 3-D printer can literally make a plastic cloak overnight,” said Yaroslav Urzhumov, assistant research professor in electrical and computer engineering at Duke University.

The new 3D-printed cloak is designed to make objects invisible to 10 GHz microwaves, which are about 3 cm (1.2 in) in wavelength. Whereas prior cloaks were made of lossy materials which prevented cloaking an object larger than a few wavelengths in size, Urzhumov's cloak is made of ABS plastic, which has very little loss at 10 GHz. In addition, ABS has an index of refraction of 1.56, meaning that similar cloaks that hide their contents from visible light can in principle be made from optical glass and plastics having micron-scale structure rather than centimeter-scale structure.

The cloak is about 3 cm thick, and cloaks a region nearly 14 cm (5.5 in) in diameter. The cloak itself is a plastic/air composite formed into an annulus about 3 cm thick that surrounds the cloaked region. The object and cloak are illuminated with radially directed microwaves. The left side of the above image shows the electric fields inside and around the cloak, while the right side shows the electric fields flowing around a solid piece of polyethylene carbonate polymer (PEC). Microwaves are approaching from the left: Deep blue indicates no electric field, dark red is the largest field.

Whereas the electric field fills the PEC, which also casts a definite shadow, essentially none of the incoming microwaves penetrate the cloak, which accomplishes this task with only minimal disturbance of the flow of microwaves. The level of invisibility can be indicated by the total proportion of microwaves scattered by the cloak in all directions. As seen in the figure above, the microwave scattering of the cloak in its working frequency (around 9.9 GHz) is about one-fifth of the amount scattered by a solid disk of ABS plastic of the same overall dimensions.

While the cloak currently only works with microwaves, the researchers believe it will be possible in the not-too-distant future to develop the technology further to work for higher wavelengths, including visible light.

"We believe this approach is a way towards optical cloaking, including visible and infrared," Urzhumov said. "And nanotechnology is available to make these cloaks from transparent polymers or glass. The properties of transparent polymers and glasses are not that different from what we have in our polymer at microwave frequencies.”

The science and technology of metamaterial-based cloaking devices is advancing in leaps and bounds. Devices such as Prof. Urzhumov's new cloak should hasten the day when such devices become integrated into consumer products.

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