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a)
A top view of the experimental setup. Microwaves enter a cavity from the
left. They fall on the composite material, consisting of alternating
rows of rods and thin copper patterned disks (1 cm in diameter). b)
A sideview of the microwaves hitting the composite material. Reported
by: Sheldon, Schulz., at the APS March Meeting in Minneapolis on March
21. |
New
Composite Material Goes Negative On Physics
Minneapolis
- March 22, 2000 - Physicists
at the University of California, San Diego have produced a new class of
composite materials with unusual physical properties that scientists theorized
might be possible, but have never before been able to produce in nature.
The remarkable
achievement, detailed in a paper that will appear in a forthcoming issue of
Physical Review Letters, was announced here today at a meeting of the American
Physical Society.
The UCSD
physicists said they expect their discovery to open up a new subdiscipline
within physics and produce an array of commercial applications for this
material, on which the university has applied for a patent.
"Composite
materials like this are built on a totally new concept," said the two
co-leaders of the UCSD team, Sheldon Schultz and David R. Smith, who announced
their discovery at a news conference.
"While
they obey the laws of physics, they are predicted to behave totally different
from normal materials and should find interesting applications," they
added.
The unusual
property of this new class of materials is essentially its ability to reverse
many of the physical properties that govern the behavior of ordinary
materials. One such property is the Doppler effect, which makes a train
whistle sound higher in pitch as it approaches and lower in pitch as it
recedes.
According to
Maxwell's equations, which describe the relationship between magnetic and
electric fields, microwave radiation or light would show the opposite effect
in this new class of materials, shifting to lower frequencies as a source
approaches and to higher frequencies as it recedes.
Similarly,
Maxwell's equations further suggest that lenses that would normally disperse
electromagnetic radiation would instead focus it within this composite
material.
This is
because Snell's law, which describes the angle of refraction caused by the
change in velocity of light and other waves through lenses, water and other
types of ordinary material, is expected to be exactly opposite within this
composite.
"If these
effects turn out to be possible at optical frequencies, this material would
have the crazy property that a flashlight shining on a slab can focus the
light at a point on the other side," said Schultz. "There's no way
you can do that with just a sheet of ordinary material."
He notes that
the development of this new class of materials, which was financed by the
National Science Foundation and the Department of Energy, is entirely
consistent with the laws of physics and was predicted as a possibility in 1968
by a Russian theorist, V.G. Veselago. "But until now," Schultz adds,
"no one had the material, so it couldn't be verified."
Underlying the
reversal of the Doppler effect, Snell's law, and Cerenkov radiation (radiation
by charged particles moving through a medium) is that this new material
exhibits a reversal of one of the "right-hand rules" of physics
which describe a relationship between the electric and magnetic fields and the
direction of their wave velocity.
The new
materials are known by the UCSD team colloquially as "left-handed
materials," after a term coined by Veselago, because they reverse this
relationship.
What that
means is physically counterintuitive -- pulses of electromagnetic radiation
moving through the material in one direction are composed of constituent waves
moving in the opposite direction.
The UCSD
physicists emphasized that while they believe their new class of composites
will be shown to reverse Snell's law, the specific composite they produced
will not do so at visible-light frequencies.
Instead, it is
now limited to transmitting microwave radiation at frequencies of 4 to 7
Gigahertz -- a range somewhere between the operation of household microwave
ovens (3.3 Gigahertz) and military radars (10 Gigahertz).
However,
Schultz said the UCSD team will soon be attempting to verify that a composite
constructed on similar principles will be able to focus and disperse
microwaves in exactly the opposite manner as normal lenses.
"We did
not do this experiment yet," he said. "But this is what the
equations predict. Physicists will understand that if our data presented in
our paper are correct, given Maxwell's equations, then this will be the
result."
The composite
constructed by the UCSD team -- which also consisted of Willie J. Padilla,
David C. Vier, and Syrus C. Nemat-Nasser -- was produced from a series of thin
copper rings and ordinary copper wire strung parallel to the rings. It is an
example of a new class of materials scientists call "metamaterials."
"Even
though it is composed of only copper wires and copper rings, the arrangement
has an effective magnetic response to microwaves that has never been
demonstrated before," said Schultz.
The idea for
the new composite came from Smith, building on the work of John Pendry of
Imperial College, London. In 1996, Pendry described a way of using ordinary
copper wires to create a material with the property physicists call
"negative electric permittivity."
Electric
permittivity -- often referred to as the "dielectric constant" -- is
the response of a material to electromagnetic radiation.
"When you
take a material like plastic, glass or sapphire and you shine microwaves onto
it, you can characterize how the microwaves going through it will behave by a
parameter called electric permittivity," explained Schultz. Most known
materials in nature have a positive electric permittivity.
Pendry also
recently suggested a way of using copper rings to make a material with
negative magnetic permeability at microwave frequencies. Just about all of the
magnetic materials in nature, those that respond to magnetic rather than
electric fields, have what physicists call a "positive magnetic
permeability."
What's unusual
about the new class of materials produced by the UCSD team is that it
simultaneously has a negative electric permittivity and a negative magnetic
permeability, a combination of properties never before seen in a natural or
man-made material.
"And the
interesting thing is that it's produced with no magnetic material," said
Schultz. "It's all done with copper."
"The
bottom line," said Smith, "is that this material -- this
metamaterial, at frequencies where both the permittivity and permeability are
negative, behaves according to a left-handed rule, rather than a right-handed
rule."
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University
of California-San Diego
Editor's
Note: The original news
release can be found at http://ucsdnews.ucsd.edu/newsrel/science/mccomposite.htm