Chalk one up for all those Star Trek fans who insist that their favorite show is really some kind of technology forecast. It turns out that cloaking devices – like the ones that hid those nasty Romulan ships from the Enterprise – may not be science fiction after all.
Researchers at Duke University’s Pratt School of Engineering this week announced that they have demonstrated a cloaking device that renders a copper cylinder nearly invisible to electromagnetic waves at certain frequencies.
Described in a paper published online byScience, this experimental device springs from an earlier design theory proposed by David Smith of Duke University and John Pendry of Imperial College London. It described an invisibility cloak made from “metamaterials” capable of redirecting different types electromagnetic waves in accordance with Maxwell's Equations. Metamaterials, which have only started to emerge over the past six years, derive their unique electromagnetic properties from their physical structures rather than the intrinsic physical properties of their constituent materials.
Duke’s real-world cloaking device consists of 10 concentric rings, the largest less than five inches across. These rings hold individual cells of metamaterials whose permeability and permittivity can be tightly controlled at any given point in the material’s structure. “There’s no way to do that with natural materials,” says David Schurig, one of the cloaking device’s creators and a physicist with Duke’s Department of Electrical and Computer Engineering.
For the cloaking device, Duke researchers relied on printed circuit technology to build a type of metamaterial known as a Split Ring Resonator (SSR). Measuring 3.33-mm across, these SSR units consists of copper rings and wires patterned onto fiberglass circuit board substrates. Controlled variations in the shape of the copper elements determine the SSR’s electromagnetic properties.
When correctly oriented in cloaking device’s concentric annular arrays, these SSRs together bend electromagnetic waves around an enclosed space and then bring them back together again. “It’s similar to a mirage,” Schurig explains. Except in this case, it’s not a temperature gradient but gradients in the material’s electromagnetic properties that allow the electromagnetic waves to bend.
Objects placed inside the enclosed space in essence become invisible with respect to those waves. Or as Schurig puts it in his Science paper, “the concealed volume plus the cloak appear to have the properties of free space when viewed externally.”
At least that’s the theory. In truth, though, the experimental cloaking device still has some limitations. Duke’s first device is a 2D set-up, in which microwaves are directed to the cloaking device via a planar waveguide. What’s more, the size, structure, and electromagnetic properties of the SSR allow it to work only with a small band of microwave frequencies.
And even at those microwave frequencies, the cloaking device doesn’t entirely hide the copper cylinder.“It still reflects a bit and leaves a bit of a shadow on the back side of the concealed object,” reports Schurig.
The cloaking device nonetheless represents an important breakthrough in the use of metamaterials--which may have a wide range of optics and electronics applications given their tunable electromagnetic properties.
According to Schurig, the cloak design is unique among metamaterials in its circular geometry and internal structural variation. All other metamaterials have been based on a cubic, or gridlike, design, and most of them have electromagnetic properties that are uniform throughout. Duke also researchers managed to come up with a design in which a single SSR handles both electrical and magnetic responses – past metamaterials often required a separate structures for the two responses.
Schurig calls the cloaking device “one of the most elaborate metamaterial structures ever produced.” And that’s all the more remarkable given that building it required only four iterations, each one taking about four or five days.
What’s more, the initial device has validated simulations of cloaking behavior that Schurig had run in Cosmol’s FEA software. “That bodes well for future designs,” he says, adding that next steps involve improving the cloaking behavior and building a 3D invisibility cloak.
As for whether Duke’s work will lead to the kind of invisibility cloak that makes objects disappear from sight, the jury is still out. Microwave frequencies are easier to attenuate than higher-frequency waves such as those that make up the visible light spectrum. Schurig notes that high-frequency metamaterials would require physical structures so small that “they fall in the realm of nanotechnology.”But Duke’s demonstration cloak represents at significant step toward devices that can control electromagnetic waves for applications ranging from military stealth devices to system that can improve radio communications.
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