Chemically assembled metamaterials pave way for superlenses

Ashley N. Paddock, ashley.paddock@photonics.com

A new metamaterial can self-assemble 3-D structures with nanoscale features, a feat that could make “superlenses” to image proteins, DNA and viruses.

Metamaterials offer new ways to manipulate light via negative refractive indices and typically are made with common deposition and lithography techniques such as electron beam lithography or atomic sputtering. But these techniques can only create materials in thin layers. The new method, proposed by Cornell University researchers led by Ulrich Wiesner, enables three-dimensional self-assembly of metamaterials.

“Today, most metamaterial fabrication relies on top-down approaches such as lithography techniques, making efficient access to three-dimensionally isotropic metamaterials challenging, thus hindering their practical application,” explained Kahyun Hur, a graduate student. “We expect that bottom-up-type metamaterial fabrication can overcome some of these limitations. In particular, block copolymer self-assembly provides a facile route to 3-D isotropic material fabrication.”

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.

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 recent paper published online in Angewandte Chemie (doi: 10.1002/ange.201104888), the researchers propose the creation of 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.

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 ranges. In addition, 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.

Special lenses made of such a material could image objects smaller than the wavelength of visible light, including proteins, DNA and viruses. Some experimenters have made such superlenses, but so far none of these 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.