May 18, 2018

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Beyond Materials: From Invisibility Cloaks to Satellite Communications

Duke-developed “metamaterials” are carefully designed structures that control all sorts of physical waves in previously impossible ways. Our researchers are poised to make these devices a household name.

When the editors of Physical Review Letters first received the manuscript, they immediately rejected it. Their form letter said the work didn’t seem important enough to even send it out for review. The editors didn’t realize that the contents would revolutionize the manipulation of electromagnetic waves that dominate today’s technology.

Then again, neither did the author.

The year was 2000, and David R. Smith was a research scientist at the University of California, San Diego, who was interested in tinkering with the properties of materials. He had a crazy notion that by controlling the structure of materials rather than the chemistry one could engineer materials with properties never seen in nature.

Smith’s imagined materials were based on collections of little conducting elements, which he had read about in papers from Sir John Pendry, a theoretical physicist at Imperial College London. By placing a series of carefully designed metal wires or rings in specific arrangements, Pendry had proposed, one could create materials with tailored and unusual electromagnetic properties.

“Just for fun, our group decided to see if this was true,” recalled Smith. “This was nothing that anyone would have funded and probably very few would have published.”

Pendry had been intrigued by the notion of materials with negative response — a condition that was well-known in optics, and which leads to a host of remarkable optical phenomena. By creating artificial materials, Pendry hoped to extend the unique properties of negative response optics across the electromagnetic spectrum.

The concept turned out to hold some water. The combination of split-ring resonators and wires which Pendry had proposed to combine negative magnetic response and negative electric response,  both behaved as predicted.

The combination of the two properties has a curious effect: It bends light backward.

This was the finding that Physical Review Letters found entirely uninteresting and not worth publishing. The rejection prompted Smith to search the literature on materials with these properties. To his surprise, a single paper popped up — one written in 1968 by a Russian physicist named Victor Veselago.

And it was a doozy.

“That paper said that if this property could ever be realized, you’d have all these crazy physics effects,” said Smith. “You’d turn light backward, you’d change the Doppler shift, Snell’s law and Cherenkov radiation — it would really be a fundamental paradigm shift. So I just changed the abstract using some of Veselago’s language and they accepted it in a snap.”

Thus began the field of metamaterials.

“The definition of metamaterials now is a little bit difficult to pin down because it’s evolved over the years,” continued Smith, who moved his research to Duke in 2004. “Our group focuses on electromagnetic properties, but a metamaterial could also mean designing acoustic or thermal properties — really anything.”

A Meta-Whatamajig?

We rely on material properties every day. Styrofoam keeps coffee hot on the morning commute. Copper carries electricity to a car’s spark plugs. Glasses bend and focus light to better see the road. These properties — thermodynamic, electrical and optical, respectively — arise due to the specific elements and arrangements of the atoms in a material.

But structures can also give material specific properties. A coil of copper wire with a magnet rotating inside will create an electric current, whereas a straight copper wire won’t. A smooth sheet of silver is reflective, while a surface coated with tiny silver spheres is black.

The Greek preposition “meta” means “beyond,” so a metamaterial is a type of material engineered to have properties beyond those provided by nature. In Smith’s and Pendry’s work, the materials receive their unusual properties through their structure rather than just their chemistry.

“Materials are made up of atoms or molecules with certain properties that create the overall properties for the material,” said Smith. “The idea of metamaterials is to duplicate that, but with artificial manmade structures that can give us material properties unlike any that exist.”


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