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  • Metamaterials Pave Way for Superlens
Nov 2011
ITHACA, N.Y., Nov. 11, 2011 — Casting tiny, oddly shaped metal structures inside plastic molds could give a boost to creating metamaterials for making “superlenses” that can image proteins, viruses and DNA.

Metamaterials, which offer new ways to manipulate light via negative refractive indices, typically are made with common deposition and lithography techniques. The limitation has been that techniques such as electron-beam lithography or atomic sputtering can create materials in thin layers only. However, researchers from Cornell University have proposed a method that uses block copolymers to self-assemble 3-D structures with nanoscale features suitable for metamaterial production.

Two polymer molecules linked together will self-assemble into a complex shape, in this case a gyroid. When one of the polymers is removed chemically, a mold remains that can be filled with metal. Finally, the other polymer is removed, leaving a metal gyroid with features measured in nanometers. (Image: Wiesner lab, Cornell)

Block copolymers are made by joining two polymer molecules at the ends so that when each end chains up with others like itself, the two solids form an interconnected pattern of repeating geometric shapes — planes, spheres, cylinders or a twisty network called a gyroid. Elements of the repeating pattern can be as small as a few nanometers across. Sometimes tripolymers can be used to create even more complex shapes.

After the structure has formed, one of the two polymers can be dissolved away, leaving a 3-D mold that can be filled with a metal. Then the second polymer is burned away, leaving a porous metal structure.

In a recently published paper in Angewandte Chemie, the Cornell researchers, led by Ulrich Wiesner, propose creating metal gyroids that allow light to pass through, but are made up of nanoscale features that interact with light. In this way, it should be possible to design materials with a negative index of refraction.

Special lenses made of such a material could image objects smaller than the wavelength of visible light, including proteins, viruses and DNA. Some experimenters have made such superlenses, but so far none that work in the visible light range. Negative-refraction materials also might be configured to bend light around an object — at least a small one — and make it invisible.

Wiesner’s team performed computer simulations of several different metal gyroids, including gold, silver and aluminum, that could be made by copolymer self-assembly, then calculated how light would behave when passing through them. They found that only silver produced satisfactory results.

They concluded that such materials could have a negative refractive index in the visible and near-infrared range. They noted that the amount of refraction could be controlled by adjusting the size of the repeating features of the metamaterial, which can be done by modifying the chemistry used in self-assembly.

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A material whose molecular structure consists of long chains made up by the repetition of many (usually thousands) of similar groups of atoms.
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