- LONDON -- US physicists have
created what can be termed as the world's first true invisibility cloak
- a device able to hide an object in the visible spectrum of light.
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- Presently, though, it works in only two dimensions and
on a tiny scale.
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- The new cloak, which is just 10 micrometres in diameter,
guides rays of light around an object inside and releases them on the other
side. The light waves appear to have moved in a straight line, so the cloak
and any object inside, appear invisible.
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- A team led by Igor Smolyaninov from the University of
Maryland, built the cloak on the basis of the first theoretical design
for an invisibility cloak, published by Vladimir Shalaev from Purdue University,
West Lafayette, Indiana, US, earlier this year.
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- According to a New Scientist report, the breakthrough
comes just a year after American and British physicists created an invisibility
cloak that worked in the microwave region of the electromagnetic spectrum.
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- At that time, a visible light cloak was thought to be
years away because of the much shorter wavelengths produced in the visible
spectrum.
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- "At optical frequencies, [wavelengths] get very
tiny, and the range of properties available from materials is limited,"
said John Pendry, a physicist at Imperial College London, and a member
of the team that produced the microwave invisibility cloak.
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- To get around this problem, Smolyaninov's team confined
light to two dimensions.
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- "The new cloak doesn't control the light you can
see directly. It's not the invisibility most people would imagine,"
said Ulf Leonhardt, a physicist at the University of St Andrews, UK.
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- As part of their experiment, the researchers injected
polarised cyan light onto a gold surface using a tiny optical fibre with
a fine tip.
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- The light waves became converted into surface plasmons
- waves rippling through the electrons of the gold surface, effectively
in two dimensions.
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- Three-dimensional invisibility cloaks would have to control
light waves both magnetically and electronically to steer them around the
hidden object. But two-dimensional surface plasmons are easier to direct.
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- "You can operate on either the electric or magnetic
channel alone," said Pendry.
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- Smolyaninov's cloak consists of a two-dimensional pattern
of concentric gold rings coated in a plastic called polymethyl methacrylate.
The plastic and the gold each have different refractive properties, and
bend plasmons in different directions. The whole arrangement lies flat
on the gold surface mentioned above.
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- By varying the mix of metal and plastic in different
areas of the cloak, Smolyaninov's team can control plasmons with enough
precision to guide them around the cloak.
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- Pendry said a simple visual analogy would be river water
flowing around a rock.
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- "It is unlikely that the cloak is perfectly invisible,
though," he said.
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- The Maryland team, however, do not report whether plasmons
can reflect off the surface of their cloak.
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- If they can, an observer could still detect its presence,
just as reflected light allows us to see a sheet of glass.
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- Smolyaninov's cloak is unlikely to be developed into
a version that works in three dimensions. But Leonhardt says it could be
useful in the near future.
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- "It could be used on computer chips. You could use
surface plasmons to communicate between different areas of a microchip,"
said Leonhardt.
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- A paper describing the new cloak is posted on the arXiv
preprint server.
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