A model of a 3D metamaterial. Ames Laboratory scientists developed a method to evaluate different conductors for use in metamaterial structures. |
Scientists at the
U.S. Department of Energy’s Ames Laboratory have designed a method to evaluate
different conductors for use in metamaterial structures, which are engineered
to exhibit properties not possible in natural materials. The work was reported in
Nature Photonics.
Cloaking devices
that hide planes from RADAR, microscopes that can see inside a single cell, and
miniature antennae that measure only a few millimeters all sound like parts of
a science fiction movie. But, within the span of the decade since they began
their work, Ames Laboratory physicist Costas Soukoulis and his research team
have moved these and other innovations from the realm of fiction closer to
reality.
“Metamaterials have a few fundamentally new
properties that may allow for many new applications,” said Soukoulis. For
instance, natural materials refract light to the opposite side of the incidence
normal, while metamaterials can refract light to the same side (left-handed
materials), allowing imaging with a flat lens. Metamaterials are also capable
of absorbing all light that hits them, reflecting none of it, creating perfect
absorbers. The materials can even slow light. And 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. But metamaterials work in the opposite direction,” said Soukoulis. “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.”
Take, for
example, the goal of creating super-efficient devices to harvest sunlight in
solar energy products. Ideal materials for such a device would absorb 100% 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. In regular materials, particularly with the types of waves
like light, materials have only an electric response, and they are always
reflective. But, in a metamaterial, we can arrange the parts of the material so
that the electric response equals the magnetic response, and the surface is
reflection free and all waves go into the material.”
Other possible
applications are “superlenses” that would allow us to use visible light to see
molecules, like DNA molecules, in detail and devices that store large amounts
of data optically. And many other potential uses exist because, unlike in
natural materials, metamaterials can be designed to work at target frequencies,
at least in principle, from radio frequencies to visible light.
But with such
great potential also comes several challenges, some of which Soukoulis’ team
have already made significant progress toward meeting. In 2006, the researchers
were the first to fabricate a left-handed metamaterial, one with a negative
index of refraction, in waves very close to visible light. In 2007, the group
designed and fabricated the first left-handed metamaterial for visible light,
and they recently fabricated chiral metamaterials that have giant optical
activity.
Another challenge
is reducing energy losses in metamaterials. Energy is lost by conversion to
heat in their metallic components. In results reported in Nature Photonics, Soukoulis and his 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,” said Soukoulis. “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.”
