Researchers have taken a step toward overcoming a key
obstacle in commercializing “hyperbolic metamaterials,” structures
that could bring optical advances including ultrapowerful microscopes,
computers, and solar cells.
The researchers have shown how to create the metamaterials
without the traditional silver or gold previously required, said Alexandra
Boltasseva, a Purdue University assistant professor of electrical and computer
Using the metals is impractical for industry because of
high cost and incompatibility with semiconductor manufacturing processes. The
metals also do not transmit light efficiently, causing much of it to be lost.
The Purdue researchers replaced the metals with an “aluminum-doped zinc
oxide,” or AZO.
“This means we can have a completely new material
platform for creating optical metamaterials, which offers important
advantages,” Boltasseva said.
Doctoral student Gururaj V. Naik provided major
contributions to the research, working with a team to develop a new
metamaterial consisting of 16 layers alternating between AZO and zinc oxide.
Light passing from the zinc oxide to the AZO layers encounters an “extreme
anisotropy,” causing its dispersion to become “hyperbolic,”
which dramatically changes the light’s behavior.
“The doped oxide brings not only enhanced performance
but also is compatible with semiconductors,” Boltasseva said.
Research findings are detailed in a paper appearing in the
Proceedings of the National Academy of
The list of possible applications for metamaterials
includes a “planar hyperlens” that could make optical microscopes 10
times more powerful and able to see objects as small as DNA; advanced sensors;
more efficient solar collectors; quantum computing; and cloaking devices.
The AZO also makes it possible to “tune” the
optical properties of metamaterials, an advance that could hasten their
commercialization, Boltasseva said.
“It’s possible to adjust the optical properties in
two ways,” she said. “You can vary the concentration of aluminum in
the AZO during its formulation. You can also alter the optical properties in
AZO by applying an electrical field to the fabricated metamaterial.”
This switching ability might usher in a new class of
metamaterials that could be turned hyperbolic and non-hyperbolic at the flip of
“This could actually lead to a whole new family of
devices that can be tuned or switched,” Boltasseva said. “AZO can go
from dielectric to metallic. So at one specific wavelength, at one applied
voltage, it can be metal and at another voltage it can be dielectric. This
would lead to tremendous changes in functionality.”
The researchers “doped” zinc oxide with aluminum,
meaning the zinc oxide is impregnated with aluminum atoms to alter the
material’s optical properties. Doping the zinc oxide causes it to behave like a
metal at certain wavelengths and like a dielectric at other wavelengths.
The material has been shown to work in the near-infrared
range of the spectrum, which is essential for optical communications, and could
allow researchers to harness “optical black holes” to create a new
generation of light-harvesting devices for solar energy applications.
The PNAS paper
was authored by Naik, Boltasseva, doctoral student Jingjing Liu, senior
research scientist Alexander V. Kildishev, and Vladimir M. Shalaev, scientific
director of nanophotonics at Purdue’s Birck Nanotechnology Center, a
distinguished professor of electrical and computer engineering and a scientific
adviser for the Russian Quantum Center.
Current optical technologies are limited because, for the
efficient control of light, components cannot be smaller than the size of the
wavelengths of light. Metamaterials are able to guide and control light on all
scales, including the scale of nanometers, or billionths of a meter.
Unlike natural materials, metamaterials are able to reduce
the “index of refraction” to less than one or less than zero.
Refraction occurs as electromagnetic waves, including light, bend when passing
from one material into another. It causes the bent-stick-in-water effect, which
occurs when a stick placed in a glass of water appears bent when viewed from
the outside. Each material has its own refraction index, which describes how
much light will bend in that particular material and defines how much the speed
of light slows down while passing through a material.
Natural materials typically have refractive indices
greater than one. Metamaterials, however, can make the index of refraction vary
from zero to one, which possibly will enable applications including the
The layered metamaterial is a so-called plasmonic
structure because it conducts clouds of electrons called “plasmons.”
“Alternative plasmonic materials such as AZO overcome
the bottleneck created by conventional metals in the design of optical
metamaterials and enable more efficient devices,” Boltasseva said.
“We anticipate that the development of these new plasmonic materials and
nanostructured material composites will lead to tremendous progress in the
technology of optical metamaterials, enabling the full-scale development of
this technology and uncovering many new physical phenomena.”