Engineers Devise Invisibility Shield
By Philip Ball - nature.com 3-1-5
Alien technology, anyone?
The idea of a cloak of invisibility that hides objects from view
has long been confined to the more improbable reaches of science
fiction. But electronic engineers have now come up with a way to
make one.
Andrea Al and Nader Engheta of the University of Pennsylvania in
Philadelphia say that a 'plasmonic cover' could render objects "nearly
invisible to an observer". Their idea remains just a proposal
at this stage, but it doesn't obviously violate any laws of physics.
"The concept is an interesting one, with several important
potential applications," says John Pendry, a physicist at Imperial
College in London, UK. "It could find uses in stealth technology
and camouflage."
Cloak of many colours
Types of invisibility shielding have been developed before, but
these mostly use the chameleon principle: a screen is coloured to
match its background, so that the screened object is camouflaged.
For example, inventor Ray Alden in North Carolina has proposed a
system of light detectors and emitters that project a replica of
the scene appearing behind an object from its front surface. Researchers
at the University of Tokyo are working on a camouflage fabric that
uses a similar principle, in which the background scene is projected
on to light-reflecting beads in the material.
But the invisibility shield proposed by Al and Engheta in a preprint
on arXiv1 is more ambitious than this. It is a self-contained structure
that would reduce visibility from all viewing angles. In that sense
it would be more like the shielding used by the Romulans in the
Star Trek episode "Balance of Terror" in 1966, which hid
their spaceships at the push of a button.
Scatter-brained
The key to the concept is to reduce light scattering. We see objects
because light bounces off them; if this scattering of light could
be prevented (and if the objects didn't absorb any light) they would
become invisible. Al and Engheta's plasmonic screen suppresses scattering
by resonating in tune with the illuminating light.
Plasmons are waves of electron density, caused when the electrons
on the surface of a metallic material move in rhythm. The researchers
say that a shell of plasmonic material will scatter light negligibly
if the light's frequency is close to the resonant frequency of the
plasmons. The scattering from the shell effectively cancels out
the scattering from the object.
For visible-light shielding, says Engheta, nature has already provided
suitable plasmonic materials: silver and gold. To reduce the scattering
of longer-wavelength radiation such as microwaves, one could make
the shield from a 'metamaterial': a large-scale structure with unusual
electromagnetic properties, typically constructed from arrays of
wire loops and coils.
Al and Engheta's calculations show that spherical or cylindrical
objects coated with such plasmonic shields do indeed produce very
little light scattering. It is as though, when lit by light of the
right wavelength, the objects become extremely small, so small that
they cannot be seen.
Size matters
Pendry warns, however, that the concept as it stands is "no
magic cloak", because it would have to be delicately tuned
to suit each different object it hides. Perhaps even more of a drawback,
he points out, is the fact that a particular shield only works for
one specific wavelength of light.
An object might be made invisible in red light, say, but not in
multiwavelength daylight.
And crucially, the effect only works when the wavelength of the
light being scattered is roughly the same size as the object. So
shielding from visible light would be possible only for microscopic
objects; larger ones could be hidden only to long-wavelength radiation
such as microwaves. This means that the technology could not be
used to hide people or vehicles from human vision.
But that need not undermine other potential uses, Engheta says.
For example, the effect could be useful for making antiglare materials.
Another possible use for plasmonic screening is microscopy, he adds.
Light microscopes could surpass their usual resolution limits by
using tiny probes to measure the light field very close to the object
being imaged. Such probes could be made 'invisible' so that they
don't disturb the imaging signal.
And of course the shielding would work fine for concealing large
objects such as spaceships from sensors or telescopes that used
long-wavelength radiation instead of visible light.
References
1. Alu A. & Engheta N. Preprint, http://arxiv.org/abs/cond-mat/0502336
(2005).
©2005 Nature Publishing Group
http://www.nature.com/news/2005/050228/full/050228-1.html
Comment
Ted Twietmeyer
3-2-5
Having worked with optical systems for some years, this article
looks like an attempt to raise funding and here's why. Especially
from the standpoint that the work is all theoretical at this stage.
Invisibility requires two things:
1. The object to be cloaked does not reflect/emit light. This is
supposedly the plasmonic concept. Note that "plasmonic"
is derived from "plasma", the 4th state of matter.
2. Scenery or other objects behind the cloaked object, must be visible
on the viewer side. Scene projection or some means to "bend"
light around the cloaked object must take place. Otherwise, if only
(1) above is employed then the object will appear as a black silhouette.
This might be fine for spacecraft or airplanes, but not too good
in broad daylight on a battle field.
The technology description does not appear to completely address
both items above. And if such technology was in the proverbial wrong
hands, perfect crimes would become commonplace.
Ted Twietmeyer |
