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 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 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 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 a switch.
Current optical technologies are limited because, for the efficient control of light, components cannot be smaller than the size of the wavelengths of light.
Imaging & Microscopy Issue 4 , 2012 as free epaper or pdf download
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.
Gururaj V. Naik, Jingjing Liu, Alexander V. Kildishev, Vladimir M. Shalaev and Alexandra Boltasseva : Demonstration of Al:ZnO as a plasmonic component for near-infrared metamaterials, PNAS, May 18, 2012
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