Researchers have been successful in developing a structure that could
bring optical advances including ultrapowerful microscopes, computers and solar
cells. They have shown how to create the metamaterials without the traditional
silver or gold previously required. 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 (AZO).
This new metamaterial consists 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 behaviour. The doped oxide brings not only enhanced
performance but also is compatible with semiconductors. Metamaterials can be
applied in optical microscopes that would make them 10 times more powerful and
able to see objects as small as DNA; and also useful in advanced sensors; more
efficient solar collectors; quantum computing; and cloaking devices. The AZO also
modulate the optical properties of metamaterials by varying the concentration
of aluminium in the AZO and also by applying an electric filed 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.
This could actually lead to a whole new family of devices that can be
tuned or switched. 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 researcher 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.
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.
Alternative plasmonic materials such as AZO overcome the bottleneck
created by conventional metals in the design of optical metamaterials and
enable more efficient devices.
