From: Alejandro Dubrovsky (s328940@student.uq.edu.au)
Date: Mon Apr 07 2003 - 14:09:51 MDT
On Tue, 2003-04-08 at 05:16, Adrian Tymes wrote:
> --- Spudboy100@aol.com wrote:
> >
> > Yes, resulting in the development of vastly,
> > improved telescopes for spysats,
> > and especially astronomical telescopes, and whatever
> > these might uncover.
> > Improved capabilities for holography, as well as
> > optical computing. Other
> > possibilities might include greatly enhanced fresnel
> > lenses for solar
> > heating, and or photovoltaics, and perhaps themionic
> > conversion. According to
> > the article, Eleftheriades and his team weren't
> > certain that what they had
> > discovered was possible, and this was last December.
> > Seems like a useful
> > tool-set. We'll see house this discovery gets used
> > and how quickly.
>
> ...it's a *lens*. A rather obvious type, it seems to
> me. Yes, lenses have all kinds of uses; this is just
> putting a lens inside a carefully-created flat sheet.
> I fail to see the significance over what we already
> had, for instance how it can significantly improve
> performance over presently available lenses in any of
> the areas you mentioned. Could you explain what I am
> failing to grasp, please?
It's not a usual lens. I (disagree/don't know enough about the physics
to agree with) Spudboy's supposed applications of the lens. The
benefits of these lenses over the standard type is that, over certain
frequency ranges, they can amplify the higher order components of light
that are only usually noticeable close to the source, letting you focus
light down to less than the usual half-a-wavelength size.
I've pasted an old (April 2001) but relevant New Scientist article
below:
Perfect focus
New Scientist vol 170 issue 2286 - 14 April 2001, page 35
A material born from one physicist's flight of fancy could revolutionise
a medical imaging technique and answer the computer industry's prayers,
says Justin Mullins
MORE than three decades ago, an obscure Russian physicist invented a
freakish material. It was not a real material that could be bent or
broken. Instead, Victor Veselago had dreamed up an imaginary substance,
and wondered what unworldly properties it might have. A harmless
pursuit, you might think, for someone working at the General Physics
Institute of the Russian Academy of Sciences in Moscow. But Veselago's
work was no an idle flight of fancy.
On the contrary, it was built on the soundest of scientific
foundations—the laws of electromagnetism laid down by James Clerk
Maxwell one hundred years before, that describe how light behaves as it
passes through any medium. Veselago's whim was to change the value of
two of the constants in Maxwell's equations from positive to negative.
He wanted to know what properties such a material might have, and also
hoped to have a little fun finding out.
He immediately realised that his creation would behave in some very
strange ways. Veselago even came to Europe in the early 1970s to promote
his ideas, but his work was regarded as little more than a curiosity.
That changed last year when physicists announced that they had finally
created his fantasy material. It is unlike any substance ever seen. It
has a structure like a distorted honeycomb and contains wires and
strange electronic components rigidly suspended in repeating patterns.
The announcement prompted a flurry of theoretical and practical work on
its properties. And amazingly, this has revealed that Veselago's
material, and others like it, are more remarkable and useful than even
he had imagined.
Over a century before Veselago's thought experiment, Maxwell had
developed his equations to describe the way electromagnetic waves such
as light pass through things like glass, water and air. But rather than
working out exactly how the two components of light—oscillating electric
and magnetic fields—vary at every point in a particular material,
Maxwell realised that it was possible to simplify the calculations by
taking a kind of average of the fields throughout the material.
He found that for a given frequency of light, this averaging introduces
a constant factor into his equations. In the case of the magnetic field,
this averaging factor is called the magnetic permeability, and for the
electric field it is the electrical permittivity. These constants are
especially useful to scientists and engineers because they describe the
overall behaviour of light as it passes through the material and can be
used to determine quantities such as the speed of light in that medium
and its refractive index—how much it bends light.
So what happened when Veselago created his imaginary substance by
changing the value of these constants to -1? Inside his fantasy
material, the usual relationship between the electric and magnetic
fields was reversed.
Light consists of an electric field and a magnetic field oscillating at
right angles to each other and to the direction in which the light is
travelling (see Diagram). Physicists have a trick for remembering how
these three are oriented, known as the right-hand rule. Arrange the
thumb, forefinger and middle finger of the right hand in such a way that
they point at right angles to each other. The thumb represents the
direction that the light is travelling, the forefinger represents the
plane of the electric field and the middle finger the plane of the
magnetic field.
In Veselago's material, however, these quantities obey a left-hand rule,
as if they were reflected in a mirror—which is why they're called
"left-handed materials".
A left-handed material should have some strange properties. It should
have a negative refractive index, for example, which means that light
entering it would bend in the opposite direction to light passing
through conventional materials. And the Doppler effect would also work
in reverse: normally the frequency of light passing through glass
increases slightly when the source is getting closer. In left-handed
materials, though, its frequency would decrease.
But Veselago never got his hands on a left-handed material. Although the
permittivity of some natural materials such as ionised gases can be
negative, the permeability of naturally occurring materials never drops
below zero. So Veselago was unable to prove his predictions with real
materials.
But in 1996, John Pendry, a theoretical physicist at Imperial College,
London, began thinking about designing new materials. He and his team
investigated the effect that a regular array of thin, parallel
conducting wires would have on a particular kind of electromagnetic
radiation—microwaves. They predicted that the average effect of the
wires would make it look as if the waves were moving through a material
with negative electrical permittivity.
Pendry reasoned that if simple wires could produce such an exotic
medium, why not arrays of other electrical components? So he began to
calculate the bulk properties of other strange concoctions. In 1999, he
published theoretical calculations for a number of other materials. One
in particular caught the eye of David Smith and his colleagues at the
University of California in San Diego.
"It was a very unusual and very unexpected effect," says Smith. What
Pendry was predicting was that a structure consisting of a periodic
arrangement of simple electronic components would have a magnetic
permeability that was negative at microwave frequencies. The electrical
components were split ring resonators—C-shaped circuits about the size
of the capital Cs in this article. Smith and his colleagues immediately
set about trying to prove Pendry's results experimentally.
Creating an array of split ring resonators is not difficult. The team
simply printed C-shaped copper circuits onto a sheet of fibreglass using
the conventional lithographic techniques used to make circuit boards.
They then cut up the sheets and stacked the slices, like a loaf of
sliced bread. But they decided they might as well go further and create
a material with both negative permittivity and permeability—a truly
left-handed material.
Thanks to Pendry's earlier work, modifying the material to give it a
negative permittivity turned out to be relatively straightforward. Smith
simply inserted arrays of fine wires into the gaps between the sheets of
resonators. Without the wires, the material absorbs microwaves at more
or less all frequencies. But when the arrays were inserted, the group
discovered that microwaves of around 5 gigahertz could pass through.
They also found evidence that the orientation of the electric and
magnetic fields was reversed, just as Veselago had predicted. The
conclusion was inescapable. Smith and his colleagues had created the
world's first left-handed material.
Their results were published last year to general acclaim, and new
studies are underway. Meanwhile, Pendry continues to make new
predictions for these materials.
Last October, he published the results of an investigation into the
electromagnetic properties of left-handed materials on a scale of a few
nanometres—in a region known as the "near field". The near field is a
kind of twilight zone close to the light source in which the electric
and magnetic fields behave wildly. On this scale there are all kinds of
components to the field that you never normally see, says Pendry. These
components are called evanescent waves and they die away rapidly as they
travel from the source, such as an excited atom in a light bulb. After a
few wavelengths the evanescent waves are so weak that physicists can
safely ignore them.
But evanescent waves cause one of the biggest headaches in optical
physics. When evanescent waves fade away, the fine detail they contain
about the source is lost. So no matter how good a lens is at focusing
light, it can never create a perfect image of the source without the
near field—there will always be an inherent "fuzziness", which means
scientists can't focus light to a spot much smaller than its wavelength.
This restriction is known as the diffraction limit and it causes
problems in all kinds of applications. For example, it limits the size
of transistors that can be carved using photolithographic techniques and
the amount of information that DVDs can store.
Pendry discovered that, in theory, left-handed materials can have a
remarkable restorative effect on evanescent waves and even focus them to
create a perfect image of the source. Nobody was more surprised at this
than Pendry himself. "It's a very unusual and surprising idea and I
didn't expect it to work," he says. But his mathematics was checked and
rechecked and the work was published in Physical Review Letters (vol 85,
p 3966).
The focusing works very differently from a conventional lens. Imagine a
light source placed just nanometres from a thin slab of left-handed
material. The source emits light, including evanescent waves, which
enter the material making electrons in it oscillate. Since the slab is
very thin, the light and evanescent waves pass through, causing
electrons on the far surface to oscillate as well. The movement of
electrons on one surface affects movement on the other, through the
electric field, and at one particular frequency they begin to resonate.
It is this resonance that amplifies the evanescent waves as they leave
the medium.
The material has a negative refractive index, so the flat slab focuses
the light like a lens, creating an image. But unlike conventional
lenses, which can't focus evanescent waves, the image is perfect—so
Pendry has coined the term superlenses to describe his slabs of
left-handed material. Of course, to catch the evanescent waves the lens
has to be placed a lot less than a wavelength from the source. For
visible light, that would be within a few tens of nanometres. This could
be a challenge, says Pendry, but it is already possible to position
objects with this kind of precision using atomic force microscopy, for
example. And if you work with radio waves, the near field stretches for
many centimetres, so the tolerances for positioning are more forgiving.
Strange materials like those made by Smith should work as superlenses
for microwaves and radio waves. In fact, Smith and a team from UCSD have
just created the first superlens with a negative refractive index for
microwaves. But Pendry reckons that a much more common material should
do the same for visible light. Metals such as silver exhibit negative
permittivity at the frequency of visible light, but have a positive
permeability. However, Pendry calculates that for evanescent waves the
permeability can be ignored. It turns out that a slab of silver just 40
nanometres thick placed 20 nanometres from the source should act as a
superlens (New Scientist, 28 October 2000, p 22). "The reason we haven't
seen this before is that nobody has looked," he says. "We're not sure
what we can do with it but we have a few ideas."
Pendry's other ideas are already finding applications. One of the most
promising is in the field of magnetic resonance imaging. MRI machines
work by placing a sample—usually a patient—inside a powerful magnetic
field. A blast of high-frequency radio waves forces nuclei in water
molecules to spin in a particular way and to re-emit radio waves at a
different, characteristic frequency. This signal is used to build a
computer image that is useful for diagnosing diseases such as cancer.
However, these machines require powerful superconducting magnets that
are expensive and bulky. The size of the machine prevents access to the
patient, so it can't be used to monitor surgery in real time.
But Pendry's superlenses could change all that. In February, Pendry and
his colleagues published the results of preliminary work in which they
tailored the properties of a left-handed material to channel radio
signals to a detector outside the machine. The team made it by rolling a
thin sheet of conducting material onto a plastic rod 20 centimetres
long, giving it a spiral cross section like a Swiss roll, and then
packing a number of these rods tightly together into a bundle. Instead
of working like a lens, the material acts like a waveguide carrying
radio waves from the nuclei in the sample to a detector. The team has
published an image of a thumb produced in this way. Being able to use
smaller detectors outside the machine should allow MRI scanners to be
smaller and cheaper. Eventually superlenses could even focus radio waves
onto tiny parts of the sample, allowing a much more detailed image to be
built up by scanning the focused radio waves across a patient, for
example.
Eventually, superlenses could also have a dramatic impact on the way
silicon chips are made. The size of transistors is currently limited by
the wavelength of light used in photolithography. Since superlenses can
focus details in a way that doesn't depend on the wavelength of light,
they magically remove this barrier and could help chip makers to build
smaller circuits. The amount of information stored on compact discs and
DVDs is also restricted by the ability of conventional lenses to focus
light to a small spot. Here, too, superlenses could help.
They could even improve the way radio antennas work. Radar beams produce
extra radiation at right angles to the main beam that create noise and
reduce the radar's efficiency. Pendry believes that superlenses could
clean up the side lobes and produce a more directed main beam. In
addition, Ely Yablonovitch at the University of California, Los Angeles,
is making a left-handed material to help reduce exposure to radiation
from mobile phones.
Pendry is convinced that these materials have an exciting future. "I'm
still at the gawping stage, but people will gradually think of
applications," he says optimistically. "They couldn't think what to do
with lasers when they were first invented—but now look at them."
Justin Mullins
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