Digital Metamaterials Get Us Closer to Invisibility Cloaks

Metamaterials can be designed to produce negative angles of refraction. If the water in the glass exhibits negative refraction, the image of the pen below the water line is flipped as if viewed in a mirror / University of Pennsylvania
Janet Fang 15 Sep 2014, 22:34

Metamaterials are man-made composites that interact with light and sound (and waves in general) in unconventional ways, resulting in exotic behavior that’s not found in nature. Now, researchers describing a new concept called “digital metamaterials” promise a simplified way of producing metamaterials—which are already being used to develop invisibility cloaks and hyperlenses that aren’t subject to the limitations of conventional materials. The work was published in Nature Materials this week.

“The idea behind metamaterials is to mimic the way atoms interact with light, but with artificial structures much smaller than the wavelength of light itself,” Boris Kuhlmey from the University of Sydney tells The Conversation. “This way, optical properties are no longer restricted to those of the constituent materials, and can be designed almost arbitrarily.”

How a metamaterial behaves is determined by the properties of its constituent units and how those subunits are arranged. Now, Cristian Della Giovampaola and Nader Engheta from the University of Pennsylvania propose using just two subunits with opposing properties. Called “metamaterial bits,” these are like the 1s and 0s in a binary code. They used nano-sized pieces of silver and silica, which interact with light in very different ways: One’s a metal, the other’s an insulator. 

The duo used a computer simulation to create layered structures that constitute bytes of increased functionality and complexity. Once they were “digitized,” the resulting material had its own unique properties, distinct from its constituent subunits.

The idea of an invisibility cloak depends on manipulating light in the way electronic circuits manipulate the flow of electrons. When electromagnetic waves (such as light) hit an object like metal or wood, they scatter; that’s how we see the object. But say there’s an artificial material that cloaks the object—bending an incoming wave around it without scattering it. Then we wouldn’t be able to see the object. 

We manipulate light all the time: magnifying glasses focus light down to a spot while mirrors reflect light and change its direction. But metamaterials can do something more sophisticated. "They’re able to bend light, to scatter it, to manipulate it in unusual ways,” Tiffany Walsh of Deakin University explains.

A pen sticking out of a glass of water wouldn’t appear like a straight line. The water surface bends the light, and light traveling from the pen to the viewer from below the water line has a different angle of refraction than the light traveling from above. However, unlike natural materials, metamaterials can produce negative angles of refraction, flipping the image (pictured above). Engheta explains the difference between the two pen-in-a-glass images in this 2013 video describing metamaterials:

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