The researchers at Purdue University used the nanoantennas to
abruptly change a property of light called its phase. Light is
transmitted as waves analogous to waves of water, which have high and
low points. The phase defines these high and low points of light.
The image in the upper left shows a schematic for an array of gold
"plasmonic nanoantennas" able to precisely manipulate light in new ways,
a technology that could make possible a range of optical innovations
such as more powerful microscopes, telecommunications and computers. At
upper right is a scanning electron microscope image of the structures.
The figure below shows the experimentally measured refraction angle
versus incidence angle for light, demonstrating how the nanoantennas
alter the refraction.
"By abruptly changing the phase we can dramatically modify how light
propagates, and that opens up the possibility of many potential
applications,"said Vladimir Shalaev, scientific director of
nanophotonics at Purdue's Birck Nanotechnology Center and a
distinguished professor of electrical and computer engineering.
Findings are described in a paper to be published online on Dec. 22 in the journal Science.
The new work at Purdue extends findings by researchers led by
Federico Capasso, the Robert L. Wallace Professor of Applied Physics and
Vinton Hayes Senior Research Fellow in Electrical Engineering at the
Harvard School of Engineering and Applied Sciences. In that work,
described in an October Science paper, Harvard researchers
modified Snell's law, a long-held formula used to describe how light
reflects and refracts, or bends, while passing from one material into
another.
"What they pointed out was revolutionary," Shalaev said.
Until now, Snell's law has implied that when light passes from one
material to another there are no abrupt phase changes along the
interface between the materials. Harvard researchers, however, conducted
experiments showing that the phase of light and the propagation
direction can be changed dramatically by using new types of structures
called metamaterials, which in this case were based on an array of
antennas.
The Purdue researchers took the work a step further, creating arrays
of nanoantennas and changing the phase and propagation direction of
light over a broad range of near-infrared light. The paper was written
by doctoral students Xingjie Ni and Naresh K. Emani, principal research
scientist Alexander V. Kildishev, assistant professor Alexandra
Boltasseva, and Shalaev.
The wavelength size manipulated by the antennas in the Purdue experiment ranges from 1 to 1.9 microns.
"The near infrared, specifically a wavelength of 1.5 microns, is
essential for telecommunications," Shalaev said. "Information is
transmitted across optical fibers using this wavelength, which makes
this innovation potentially practical for advances in
telecommunications."
The Harvard researchers predicted how to modify Snell's law and demonstrated the principle at one wavelength.
"We have extended the Harvard team's applications to the near
infrared, which is important, and we also showed that it's not a single
frequency effect, it's a very broadband effect," Shalaev said. "Having a
broadband effect potentially offers a range of technological
applications."
The innovation could bring technologies for steering and shaping
laser beams for military and communications applications, nanocircuits
for computers that use light to process information, and new types of
powerful lenses for microscopes.
Critical to the advance is the ability to alter light so that it
exhibits "anomalous" behavior: notably, it bends in ways not possible
using conventional materials by radically altering its refraction, a
process that occurs as electromagnetic waves, including light, bend when
passing from one material into another.
Scientists measure this bending of radiation by its "index of
refraction." Refraction 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. All
natural materials, such as glass, air and water, have positive
refractive indices.
However, the nanoantenna arrays can cause light to bend in a wide range of angles including negative angles of refraction.
"Importantly, such dramatic deviation from the conventional Snell's
law governing reflection and refraction occurs when light passes through
structures that are actually much thinner than the width of the light's
wavelengths, which is not possible using natural materials," Shalaev
said. "Also, not only the bending effect, refraction, but also the
reflection of light can be dramatically modified by the antenna arrays
on the interface, as the experiments showed."
The nanoantennas are V-shaped structures made of gold and formed on
top of a silicon layer. They are an example of metamaterials, which
typically include so-called plasmonic structures that conduct clouds of
electrons called plasmons. The antennas themselves have a width of 40
nanometers, or billionths of a meter, and researchers have demonstrated
they are able to transmit light through an ultrathin "plasmonic
nanoantenna layer" about 50 times smaller than the wavelength of light
it is transmitting.
"This ultrathin layer of plasmonic nanoantennas makes the phase of
light change strongly and abruptly, causing light to change its
propagation direction, as required by the momentum conservation for
light passing through the interface between materials," Shalaev said.
The work has been funded by the U.S. Air Force Office of Scientific
Research and the National Science Foundation's Division of Materials
Research.
From sciencedaily
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