The researchers have shown how to create the metamaterials without the traditional silver or gold previously required, says Alexandra Boltasseva, assistant professor of electrical and computer engineering.

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 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 says.

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 says.

Research findings are detailed in a May 14 article in the Proceedings of the National Academy of Sciences.

Exciting applications

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.

A researcher can design so-called hyperbolic metamaterials, which consist of alternating layers of metal-dielectric systems (lower left), such as AZO/ZnO, GZO/ZnO, TiN/AlN, Au/Al2O3, and Ag/Al2O3. The simulated performance characteristics, or figures of merit (right), and examples of detecting subwavelength features (upper left) reveal the potentially powerful resolving power of such metamaterials.

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(Image courtesy of Alexandra Boltasseva)

The AZO also makes it possible to tune the optical properties of metamaterials, an advance that could hasten their commercialization, Boltasseva says. “It’s possible to adjust the optical properties in two ways. 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 nonhyperbolic at the flip of a switch.

“This could actually lead to a whole new family of devices that can be tuned or switched,” Boltasseva says. “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, the Bob and Anne Burnett Distinguished Professor of Electrical and Computer Engineering and a scientific adviser for the Russian Quantum Center.

Improving on nature

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 hyperlens.

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 says. “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.”

This work has been funded in part by the U.S. Office of Naval Research, National Science Foundation and Air Force Office of Scientific Research.