INSIGHTS | PERSPECTIVES
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For centuries, magicians have created the illusion of invisibility with the as- sistance of smoke and mirrors: Mir- rors redirect light and fool our eyes; smoke conceals the trickery. The con- cept of redirecting light has become
routine in the ongoing development of “
invisibility cloaks”—constructs of specially designed materials that can route light around
an object, making it appear as if both the
object and cloak are empty space. On page
427 of this issue, Schittny et al. (1) show how
to use the other half of the magicians’ tool
kit, showing that objects in a smoky or “
diffusive” environment can be perfectly hidden
from sight.
Although the challenges in creating an
invisibility cloak are substantial, key advances in artificial materials—or metamaterials—during the past decade kept alive the
dream of true invisibility. The requirements
for an optical cloak are indeed stringent. A
ray of light entering a cloak must not be reflected and must emerge from the cloak as
if having passed through an equivalent region of empty space. A handful of theorems
require that such a cloak must consist of a
material with both electric and magnetic
response, be anisotropic, and have properties that vary continuously throughout the
cloak volume. Naturally occurring materials
do not have enough flexibility to meet these
requirements.
Motivated by the increasingly complex
materials that were becoming achievable
using metamaterials, Pendry et al. (2) sug-
gested a new design tool, transformation
optics, for cloaking. The propagation of light
and other electromagnetic waves through
a medium is described by Maxwell’s equa-
tions; the material enters solely through two
parameters: the electric permittivity, ε, and
the magnetic permeability, µ. A conceptu-
ally simple coordinate transformation to
Maxwell’s equations produces an exact speci-
fication for a cloak, which can be represented
as values of ε and µ that vary throughout the
cloak structure. Schurig et al. (3) used this
approach to demonstrate a metamaterial in-
visibility cloak at microwave frequencies.
While transformation optics allows
for a perfect optical cloak, this cloak is
ultimately limited by fundamental phys-
ics. Cloaking requires that a light ray
traveling through the cloak arrive at the ob-
server at the same time as a ray that would
have passed through empty space. The di-
verted ray takes a longer path and would
need to travel faster than the speed of light
in vacuum, which Einstein’s theory of spe-
cial relativity rules out. A slight loophole
in this limitation allows the ideal cloak to
work over a very small bandwidth. Other
types of wave phenomena do not possess
the inviolable speed-of-light limitation.
Transformation optical cloaks for sound
waves, for example, can be much more eas-
ily realized, as recently demonstrated by
Zigoneanu et al. (4).
If the object to be cloaked is much
smaller than the wavelength of light, then
the light need not be diverted to such an
extent, and one might imagine cloaking
would be considerably easier. Indeed, Alù
and Engheta (5) suggested that a simple
material coating could be applied to small
objects in such a way that the composite
would not scatter light. This approach of
a material coating is obviously limited
because subwavelength objects are effec-
tively beyond the diffraction limit and not
visually detectable anyway. However, the
coating idea works for other physical phe-
nomena that are not waves and do not have
a wavelength as a parameter. To manage
heat flow, for example, Xu et al. recently
showed that a material with an appropri-
ately chosen thermal conductivity could be
fashioned into an extremely thin coating to
achieve cloaking-like behavior—in this in-
stance, keeping a desired area of a heated
plate cool (6).
The idea that a simple coating could
work as a cloak for heat flow and other
physical phenomena prompted Schittny et
al. to take another look at optical cloaking.
Although light traveling in empty space is
governed by the wave equation, light traveling through a fog or similarly dense, murky
medium no longer travels in a straight
A cloaking
coating for
murky media
By David R. Smith
Objects shrouded in fog
disappear entirely with
simple coating
METAMATERIALS
Center for Metamaterials and Integrated Plasmonics
and Department of Electrical and Computer Engineering,
Duke University, Durham, NC 27708, USA.
E-mail: drsmith@ee.duke.edu
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R2
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Cloaking in diffusive media. In a smoky or other strongly scattering medium, light propagates by bouncing
randomly from one particulate to the next. As a result, light transport is better described by the mechanism of
diffusion as opposed to wave mechanics (as shown on the right). In this diffusive regime, an invisibility cloak can be
created by simply wrapping the object (with radius R1) to be hidden with a thin shell of homogeneous composite
material (which increases the radius to R2). The trick developed by Schittny et al. is to select the diffusion constant
of the cloak material in such a way that it perfectly cancels any scattering from the underlying object.
