A lens made out of identical metamaterial bytes (above) can be made flat by altering the composition of the bytes (below).Headlines touting invisibility cloaks, flat lenses and other optical devices based on metamaterials have been hyped for many years. Now, researchers at the University of Pennsylvania have discovered a way to simplify the design of metamaterials so that almost any desired permittivity can be obtained by combining two materials, one with an appropriate positive, the other with an appropriate negative permittivity. [Della Giovampaola and Engheta, Nature Mater (2014) 13, 1115-1121 DOI:10.1038/nmat4082]
Nader Engheta and colleagues explain the concept by borrowing from binary computing so that “digital” metamaterials composed of metamaterial “bits” are combined into “bytes” that can have different shapes. For instance, nanoscale cylinders consisting of one metamaterial bit wrapped in a shell of the other could be constructed. By altering the radii of the cores and shells, as well as which of the two bits is the interior and which the outer, the researchers showed theoretically the bulk metamaterial could be tuned to any given permittivity value. They used glass and silver in their calculations but the combinations of different materials are almost limitless so could be selected for particular applications and layered or other architectures might also be exploited.
“The inspiration came from digital electronics,” Engheta explains. “With binary systems, we can take an analog signal - a wave - and sample it, 'discretize' it and ultimately express it as a sequence of zeroes and ones. We wanted to see if we could break down a material’s electromagnetic properties in the same way." Unfortunately, combining materials randomly may not work. Combining properly a material with a permittivity of 2 and a second with -4 might produce a new materials with a permittivity of 30 rather the average of the two starting materials, the tuning is down to how the materials are combined, their relative arrangement and geometry. By creating overarching structures of these "bytes", their calculations show how flat lenses, hyperlenses and waveguides might be made.
As an example Engheta explains that, “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,” he says. “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 has that value.” A given optical application then becomes a matter of selecting the appropriate materials and arranging them as per the instructions generated by their model to give the desired properties. It might thus be possible to make digital metamaterial hyperlenses for a microscope that can resolve at distances shorter than the wavelength of light or waveguides that can channel light around corners and so ultimately give us that much hyped invisibility cloak.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".