Researchers using nanotechnology
have taken a step toward creating an "optical
cloaking" device that could render objects invisible
by guiding light around anything placed inside this
"cloak."
The Purdue University engineers, following
mathematical guidelines devised in 2006 by physicists
in the United Kingdom, have created a theoretical
design that uses an array of tiny needles radiating
outward from a central spoke. The design, which
resembles a round hairbrush, would bend light around
the object being cloaked. Background objects would be
visible but not the object surrounded by the
cylindrical array of nano-needles, said Vladimir
Shalaev, Purdue's Robert and Anne Burnett Professor of
Electrical and Computer Engineering.
The design does, however, have a major limitation:
It works only for any single wavelength, and not for
the entire frequency range of the visible spectrum,
Shalaev said.
"But this is a first design step toward creating an
optical cloaking device that might work for all
wavelengths of visible light," he said.
Research findings are detailed in a paper appearing
this month in the journal Nature Photonics. The paper,
which is appearing online this week, was co-authored
by doctoral students Wenshan Cai and Uday K. Chettiar,
research scientist Alexander V. Kildishev and Shalaev,
all in Purdue's School of Electrical and Computer
Engineering.
Calculations indicate the device would make an
object invisible in a wavelength of 632.8 nanometers,
which corresponds to the color red. The same design,
however, could be used to create a cloak for any other
single wavelength in the visible spectrum, Shalaev
said.
"How to create a design that works for all colors
of visible light at the same time will be a big
technical challenge, but we believe it's possible," he
said. "It is clearly doable. In principle, this cloak
could be arbitrarily large, as large as a person or an
aircraft."
The research is based at the Birck Nanotechnology
Center at Purdue's Discovery Park.
Other researchers published findings in 2006
describing the mathematics generally required for the
optical cloaking device. Those researchers include:
John Pendry at the Imperial College in London, along
with David Schurig and David R. Smith at Duke
University, and simultaneously, Ulf Leonhardt at the
University of St. Andrews in Scotland.
"These mathematical requirements were very general,
and then we determined how to fulfill the requirements
with a specific design," Shalaev said.
Leonhardt, a professor of theoretical physics,
wrote a commentary piece about the Purdue paper
appearing in the same issue of Nature Photonics. In
the commentary, he compares the Purdue design to the
Roman creation of "the first optical metamaterial," a
type of glass containing nanometer-scale particles of
gold. In ordinary daylight, a cup made of the glass
appeared green, but then it glowed ruby when
illuminated from the inside.
The Purdue research, Leonhardt writes, represents "
... theoretical simulations that show that a modified
Roman cup based on modern nanofabrication technology
will act as an invisibility device ... Any object you
put inside will disappear as if dissolved in air,
provided it is viewed through polarizing tinted
glasses of precisely that colour."
Other researchers have developed concepts for
cloaking objects smaller than the wavelengths of
visible light and for objects detected in the
microwave range of the spectrum, which are much larger
than the wavelengths of visible light. But the new
design is the first for cloaking an arbitrary object
in the range of light visible to humans.
"What we propose is the cloaking of objects of any
shape and size," Shalaev said.
Two requirements are needed to render an object
invisible: Light must not reflect off of the object,
and the light must bend around the object so that
people would see only the background and not the
cloaked object itself.
"If you satisfied only the first requirement of
preventing light from reflecting off of the object,
you would still see the dark shadowlike shape of the
object, so you would know something was there,"
Shalaev said. "The most difficult requirement is to
bend light around the cloaked object so that the
background is visible but not the object being
cloaked. The viewer would, in effect, be seeing
around, or through, the object."
The device would be made of so-called "non-magnetic
metamaterials." Meta in Greek means beyond, so the
term metamaterial means to create something that
doesn't exist in nature. Unlike designs for
invisibility in the microwave range, the new design
has no magnetic properties. Having no magnetic
properties makes it much easier to cloak objects in
the visible range but also causes a small amount of
light to reflect off of the cloaked object.
"But this could, in principle, be offset by other
means, for example, with antireflective coatings,"
Shalaev said. "The big challenge is how to make rays
bend around the object, which we have described how to
do in this paper."
A key factor in the design is the ability to reduce
the "index of refraction" to less than 1. Refraction
occurs as electromagnetic waves, including light, bend
when passing from one material into another.
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
and defines how much the speed of light slows down
while passing through a material.
Natural materials typically have refractive indices
greater than 1. The new design reduces a refractive
index to values gradually varying from zero at the
inner surface of the cloak, to 1 at the outer surface
of the cloak, which is required to guide light around
the cloaked object.
Creating the tiny needles would require the same
sort of equipment already used to fabricate nanotech
devices. The needles in the theoretical design are
about as wide as 10 nanometers, or billionths of a
meter, and as long as hundreds of nanometers. They
would be arranged in layers emanating from a central
spoke in a cylindrical shape. A single nanometer is
roughly the size of 20 hydrogen atoms strung together.
Although the design would work only for one
frequency, it still might have applications, such as
producing a cloaking system to make soldiers invisible
to night-vision goggles.
"Because night-imaging systems detect only a
specific wavelength, you could, in theory, design
something that cloaks in that narrow band of light,"
Shalaev said.
Another possible application is to cloak objects
from "laser designators" used by the military to
illuminate a target, he said.
Leonhardt says in his commentary that creating a
cloak for rendering total invisibility in the entire
visible spectrum would require "further advances in
optical metamaterials, new combinations of
nanotechnology with highly abstract ideas ..."
The optical cloaking research is an indirect
spinoff of research in Shalaev's lab that has been
funded by the U.S. Army Research Office to develop
metamaterials. In previous work, Shalaev's team
created a metamaterial that has a "negative index of
refraction" in the wavelength of light used for
telecommunications, a step that could lead to better
communications and imaging technologies. More
recently, the researchers moved the wavelength for a
negative refractive index material to the visible
range.
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Published on 30 April 2007
