New Candidates Sought for Metamaterial Conductors

A new method that evaluates different conductors for use in metamaterial structures could bring closer to reality super-efficient solar energy devices and superlenses that allow us to use visible light to see molecules like DNA.

Metamaterials are engineered to exhibit properties not possible in natural materials. While natural materials refract light to the opposite side of the incidence normal, metamaterials can refract light to the same side (left-handed material), allowing imaging with a flat lens. They also are capable of absorbing all light that hits them, reflecting none of it and creating perfect absorbers. The material can even slow light.

But what makes these properties even more interesting is that they can be adjusted to the needs of particular technologies.

“Usually, materials scientists are presented with a material, determine its properties and only then come up with a use for the material,” said physicist Costas Soukoulis of Ames Laboratory. “But metamaterials work in the opposite direction. With metamaterials, we can think about what technology we’d like and what properties we want — perhaps properties unheard of before — and design the materials to exhibit those properties.”

A model of a 3-D metamaterial structure. Metamaterials are engineered materials that exhibit properties not possible in natural materials, such as reflecting light to the same side as incidence normal, absorbing all light that hits them, and slowing light. (Image: US Department of Energy’s Ames Laboratory)

Take, for example, the goal to create super-efficient devices to harvest sunlight in solar energy products. It would be ideal if the material of such a device could absorb 100 percent of the solar spectrum.

"In metamaterials, we can design both their magnetic and electric responses," said Thomas Koschny, Ames Laboratory associate scientist. "Therefore, we can control the reflection at the interface of the metamaterial, which you cannot easily do in normal materials.”

Regular materials, particularly with the types of waves like light, are always reflective and have only electric responses. Metamaterials, on the other hand, can be arranged so that the electric response equals the magnetic response, and the surface can become reflection free, with all lightwaves being absorbed into the material, Koschny said.

Devices that store large amounts of data optically are among the other possible applications. Many other potential uses exist because, unlike natural materials, metamaterials can be designed to work at target frequencies, at least in principle, from radio frequencies to visible light.

Several challenges lie ahead, some of which Soukoulis’ team has already made significant progress toward meeting.

The researchers successfully fabricated a left-handed metamaterial, one with a negative index of refraction, in waves very close to visible light (See: Metamaterials Used to Alter Light's Path, Speed); they later created the first left-handed metamaterial for visible light. Most recently, the team fabricated chiral metamaterials that have giant optical activity.

To address metamaterials’ inability to keep energy, which they lose through heat conversion in their metallic components, Soukoulis’ team evaluated a variety of conducting materials, including graphene, high-temperature superconductors and transparent conducting oxides.

"Graphene is a very interesting material because it is only a single atom thick, and it is tunable, but unfortunately it does not conduct electrical current well enough to create an optical metamaterial out of it," said Philippe Tassin, a postdoctoral research associate at Ames Laboratory. "We also thought high-temperature superconductors were very promising, but we found that silver and gold remain the best conductors for use in metamaterials."

While neither graphene nor superconductors will immediately fix losses in metamaterials, Soukoulis’ work provides a method for evaluating future candidates to replace gold or silver that will help harness the enormous potential of metamaterials.

"Metamaterials may help solve the energy problems America is facing," Soukoulis said. "There's no shortage of new ideas in the field of metamaterials, and we’re helping make progress in understanding metamaterials' basic physics, applied physics and possible applications."

The work appeared in Nature Photonics.

For more information, visit: www.ameslab.gov