From the archive, originally posted by:  [ spectre ]

Two prescriptions for an invisibility cloak have been unveiled by
physicists in the United Kingdom and the United States. The researchers
say that in principle the technologies needed for building these
devices already exist. “Invisibility is visibly close,” says Ulf
Leonhardt of the University of St Andrews in Scotland, one of the
researchers behind the proposals.

He and John Pendry of Imperial College London, UK, and their co-workers
have independently described similar ways to create an invisible ‘hole’
in space, inside which objects can be hidden. They say it is possible
to guide light around the hole, rather like water flowing around a rock
in a river, so that the object inside it cannot be seen.

Light rays are bent when they pass between materials with different
refractive indices, such as air and water. But bending light so that it
passes round a region of space and emerges travelling along the same
line as it was initially is a difficult trick, requiring an
invisibility cloak made from materials with a ‘tunable’ refractive

Such substances have been made, in the form of so-called metamaterials.
These are built from rings or coils of metal wire, etched into printed
circuit boards and glued together, which act as antennae that interact
with the electromagnetic field of incoming light and modify the paths
the light takes. Such metamaterials can have bizarre optical
properties: for example, having negative refractive index, so that they
bend light the ‘wrong’ way.

Leonhardt and Pendry’s teams have shown how in theory metamaterials
with ‘sculpted’ optical properties could be deployed to guide light
rays around an object.

Light in the hole

This isn’t the first time metamaterials have been proposed as the
fabric of an invisibility cloak.

But in previous work, each object would need a cloak specially tailored
for it. In contrast, the methods suggested by Leonhardt and by Pendry
and colleagues allow anything to be hidden inside the hole.

The maths governing the behaviour of light shows that it’s impossible
for a material to completely evade scattering or absorbing any light
whatsoever, which means it cannot be perfectly invisible.

But Leonhardt says that the imperfections can be made so small that
they’ll barely matter. “Maybe the device would create a slight haze,”
he says.

The ability to conceal a tank or an aircraft would obviously be
welcomed by generals everywhere; current stealth aircraft simply aim to
eliminate radar reflections, making them ‘black’ rather than truly
invisible. “I have reason to suspect that the military is already
working on such devices,” says Leonhardt.

Dark magic

Leonhardt compares his theoretical invisibility cloak to something like
a black hole: while some light rays passing close by the ‘optical hole’
will be bent around it, others travelling on certain trajectories would
get trapped inside. The tricky bit, he says, is that one should then
“gently guide the light back from the optical underworld.” He says that
this could be done by suitably shaping the refractive index changes of
the space inside the hole, giving it properties like those of special
lenses used in radar technology.

Both cloaking devices would work only for wavelengths close to the size
of the individual components of the metamaterial. For visible light,
this would mean that the structures would have to be fashioned using
micro- and nano-engineering.

But Pendry and colleagues believe it should be possible to achieve
invisibility over a reasonably broad band of wavelengths (over the
entire visible spectrum, say) by embedding their ‘optical hole’ in a
material with a high refractive index.

There’s no way to know yet if either design can be practically built.
But Leonhardt says that Pendry’s approach suggests ways to avoid some
of the most demanding technical challenges of his own design. “I only
just saw their paper,” says Leonhardt, “and since then I am buzzing
with ideas about how to combine the two different strategies.”


Submitted on February 7, 2006
Accepted on April 26, 2006

Controlling Electromagnetic Fields
J. B. Pendry 1*, D. Schurig 2, D. R. Smith 2

1 Department of Physics, Blackett Laboratory, Imperial College London,
London SW7 2AZ, UK.
2 Department of Electrical and Computer Engineering, Duke University,
Box 90291, Durham, NC 27708, USA.

* To whom correspondence should be addressed.
J. B. Pendry , E-mail: j [dot] pendry [at] imperial [dot] ac [dot] uk

Using the freedom of design that metamaterials provide, we show how
electromagnetic fields can be redirected at will and propose a design
strategy. The conserved fields–electric displacement field D, magnetic
induction field B, and Poynting vector S–are all displaced in a
consistent manner. A simple illustration is given of the cloaking of a
proscribed volume of space to exclude completely all electromagnetic
fields. Our work has relevance to exotic lens design and to the
cloaking of objects from electromagnetic fields.

Submitted on February 21, 2006
Accepted on April 26, 2006

Optical Conformal Mapping
Ulf Leonhardt 1*

1 School of Physics and Astronomy, University of St Andrews, North
Haugh, St Andrews KY16 9SS, Scotland.

* To whom correspondence should be addressed.
Ulf Leonhardt , E-mail: ulf [at] st-andrews [dot] ac [dot] uk

An invisibility device should guide light around an object as if
nothing were there, regardless of where the light comes from. Ideal
invisibility devices are impossible due to the wave nature of light.
This paper develops a general recipe for the design of media that
create perfect invisibility within the accuracy of geometrical optics.
The imperfections of invisibility can be made arbitrarily small to hide
objects that are much larger than the wavelength. Using modern
metamaterials, practical demonstrations of such devices may be
possible. The method developed here can be also applied to escape
detection by other electromagnetic waves or sound.