New Theory: How to Make Objects Invisible
> New Theory: How to Make Objects Invisible
> By Robert Roy Britt
> LiveScience Senior Writer
> posted: 28 February 2005
> High-tech cloaking machines could one day render very small objects nearly
> invisible and perhaps improve military stealth technology, scientists said
> The idea is straight out of science fiction - cloaking technology made
> Romulan spaceships disappear in Star Trek. A humble version of the device
> could become a reality, according to Nader Engheta and Andrea Alu of the
> University of Pennsylvania.
> But don't expect to hide yourself or your spaceship anytime soon, at least
> not in the standard sense of invisible. In practical terms, the research
> is more likely to lead to improved technical and research devices, and
> even these applications are years away.
> How it would work
> The proposal involves using plasmons - tiny electronic excitations on the
> surfaces of some metals - to cancel out the visible light or other
> radiation coming from an object.
> "A proper design . may induce a dramatic drop in the scattering
> cross-section, making the object nearly invisible to an observer," Nader
> and Alu write in a scientific paper
> [http://arxiv.org/abs/cond-mat/0502336%5d that was made available to the
> public Feb. 14.
> But cloaking ability would depend on an object's size, so that only with
> very small things - items that are already microscopic or nearly so -
> could the visible light be rendered null. A human could be made impossible
> to detect in longer-wavelength radiation such as microwaves, but not from
> visible light.
> A spaceship might be made transparent to radio waves or some other
> long-wavelength detector.
> The idea is in an infant stage but appears not to violate any laws of
> physics, according to an article Monday in news@..., an online
> companion to the journal Nature, which provided advance copies of the
> story to reporters.
> "The concept is an interesting one, with several important potential
> applications," John Pendry, a physicist at Imperial College in London in
> the UK, told the publication. "It could find uses in stealth technology
> and camouflage."
> But Engheta, co-developer of the idea, said such applications can't even
> be considered yet.
> "Things like airplanes are very complex objects - complex shape and
> complex materials - and I do not know to what extent our concept can be
> applicable to that," Engheta told LiveScience. "We are still in the
> conceptual stage, and there are several important questions that have to
> be answered before any practical scenario can be considered."
> Plasmons are real
> You've seen cloaking technology at work on television, when blue
> backgrounds are used to make a person invisible. Alu and Engheta envision
> something far more sophisticated.
> Objects are visible in the optical range because they reflect light, a
> process scientists call scattering. Objects absorb light, too, and what is
> absorbed is not seen. The sky is blue because the atmosphere scatters blue
> light more than red.
> A plasmonic cloaker would resonate with a particular wavelength of light,
> so that the wavelength would not scatter.
> Plasmons are real, a product of a strange characteristic of light, which
> is made up of both particles and waves. Plasmons are created when
> electrons on the surface of a metallic material move in rhythm. They have
> other odd properties.
> Back in 1998, researchers led by Thomas Ebbesen of the Louis Pasteur
> University in Strasbourg, France shone light on a sheet of gold foil that
> contained millions of tiny holes. The holes were smaller than the
> wavelength of the light, and Ebbesen expected no light to get through.
> Amazingly, more light came out the other side than what hit the holes.
> Follow-up research found that plasmons - jittery little waves on the
> surface of the metal - were snagging light and stuffing it through the
> holes. "When the energy and momentum of the photons match the energy and
> momentum of the plasmons, the photons are absorbed and radiated again on
> the other side," according to an article in the May 1998 edition of
> Photonics Spectra magazine.
> Reality sets in
> Engheta and Alu say objects coated with perhaps loops or coils of silver
> or gold might do the trick.
> But there are many hurdles. It is not clear how even a small object could
> disappear in daylight, which itself contains many different wavelengths,
> or colors, of light. Presumably a plasmonic device would have to be built
> to cloak each wavelength.
> Anything not perfectly ball-shaped presents additional problems. The
> researchers' calculations suggest "homogeneous spherical objects" in the
> nanoscale range - really, really small - could be rendered optically
> Practically speaking, the technology, if developed, might be used in
> antiglare materials or to improve microscopic imaging in about five years,
> Engheta said.