Penn research shows way to design 'digital' metamaterials

This phenomenon is critical to the design principles behind digital metamaterials bytes. For a certain set of metamaterial bits, when the material with positive permittivity (typically a dielectric) is on the inside, the permittivity of the byte ranges between the values of two the materials. When the material with negative permittivity (typically a metal) is on the inside, however, the overall value varies widely outside that range. Fine-tuning the ultimate permittivity of a byte then entails altering the thickness of each of the materials.

For simplicity's sake, the researchers simulated metamaterial bytes made out of silver and glass in their study, but stressed that any pair of materials that followed the negative/positive rule would work.

"If I want a metamaterial with permittivity of 14, I can pick any two materials, as long as one is positive and one is negative, and select them based on the other properties I need for my application," Engheta said. "Silver and glass, for example, might not have the right mechanical or thermal properties for what I want to do, so I can select other materials and get to the permittivity I need by altering the radii and order of them in the metamaterial byte."

"This gives us a lot of flexibility," he said. "It's just like how I can select the voltage I want to represent a '1' in an electronic circuit. If it's a regular circuit in the lab, a '1' might be 5 volts, but if it's a nanoscale device, I might want to have a '1' be 5 microvolts."

The researchers selected the core-shell geometry of the bytes because it is a structure that materials scientists are already adept at constructing. Alternate byte geometries, such as ones constructed out of alternating layers of the two materials, are possible.

Once bytes are constructed, the way they are arranged in proximity to each other enables various optical applications.

"If we wanted to make a lens with a permittivity of 4, but didn't have a single material with that value, we could take any two materials with the positive/negative rule and design bytes such that they each have a permittivity of 4," Engheta said. "If we arrange them together in the shape of the lens, the whole thing looks like it has a permittivity of 4 from the perspective of a light wave, even though none of the materials in it have that value."

"We can take it a step farther, and make a flat lens that focuses light in the same way," he said. "We could arrange bytes in a layers, but instead of their height changing, we change their permittivity so that it bends the wave in a manner expected from the lens."

With the ability to spatially vary the permittivity of a metamaterial in such a discrete way, other optical applications are just a matter of the proper arrangement. The researchers demonstrated the feasibility of digital metamaterial hyperlenses, which can image things smaller than the wavelength of light, as well as waveguides that channel light around curves and corners. Carefully arranged such that they channel light around an object, such waveguides would create the illusion of light passing through the object unimpeded, effectively rendering it invisible.

Nader Engheta, the H. Nedwill Ramsey professor of Electrical and Systems Engineering in Penn's School of Engineering and Applied Science, explains the basic premise behind metamaterials, and how they achieve electromagnetic properties not found in nature.

(Photo Credit: University of Pennsylvania)

A metamaterial with a given permittivity can be designed out of any two materials, called 'metamaterial bits,' so long as the permittivity of one of the materials is positive and the other is negative. Borrowing terms from binary computing, these 'digital' metamaterials are composed of metamaterial 'bits,' which are combined into 'bytes.'A lens made out of identical metamaterial bytes (above) can be made flat by altering the composition of the bytes (below).

(Photo Credit: University of Pennsylvania)

Source: University of Pennsylvania