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a.m. Pacific Time (2 p.m. ET)
Media
Contact: Kim McDonald (858) 534-7572, kimmcdonald@ucsd.edu
David
Smith (858) 534-1510, drs@sdss.ucsd.edu
Photograph
of composite available at: http://ucsdnews.ucsd.edu/newsrel/science/mcreversed.htm
Credit:
Richard A. Shelby, UCSD
Diagram
illustrating composite material’s properties available at: http://physics.ucsd.edu/lhmedia
Credit:
Richard A. Shelby, UCSD
Physicists at the University
of California, San Diego who last year produced a new class of composite
materials believed to reverse the behavior of many fundamental electromagnetic
properties associated with materials, have experimentally verified the first of
these predicted reversals.
Their experiments, detailed
in the April 6 issue of Science,
demonstrate that electromagnetic radiation travels through the composite
material in a manner never before seen in nature.
The achievement is much more than a physical curiosity. The new
material could prove useful in the development of novel antennas and other
electromagnetic devices. It may also make possible the construction of a
“perfect lens,” capable of focusing light and other forms of radiation to
limits not achievable by normal lenses.
In their experiments, the UCSD physicists built a sample of their
material small enough to fit in a hand out of fiberglass and tiny copper wires.
They then sent
microwaves through it—at the same frequency as those
used in police radar guns. What they discovered is exactly what was predicted a
year ago—that the microwaves would emerge from the sample in a direction
predicted by Snell’s law, assuming a negative index of refraction, which describes the angle of
refraction produced by the slowing of light and other forms of electromagnetic
radiation through water, glass and other ordinary material.
“The experiments we report confirm earlier theoretical predictions
that a new, unique class of materials can cause electromagnetic waves, such as
radar and microwaves, to bend in a direction opposite to the way they travel
through all other known materials,” says Sheldon Schultz, a professor of
physics at UCSD. He reported the advance with UCSD physicists David R. Smith
and Richard A. Shelby.
“If these effects turn out to be possible at optical frequencies, this material would have the crazy property that a small flashlight shining on a flat slab would produce a focus at a point on the other side,” says Schultz. “There’s no way you can do that with just a flat sheet of ordinary material.”
Physicists measure the
bending of light, microwaves and other forms of radiation through a material by
its “index of refraction.” The bigger a material’s index, the slower light
travels through it, and the more it “bends,” or changes direction when going
from one material to a different one. Air, for example, has a refractive index
of 1.0 for light; water, 1.3; and glass, about 1.5. This means that a beam of
light passing from air to water is deflected in one direction by a certain
amount and is deflected by glass by a slightly greater angle in the same
direction. This bending, in combination with the curved glass surfaces, is what
allows lenses to focus their light.
Electromagnetic radiation
traveling through ordinary materials is always deflected in the same direction,
giving those materials a “positive index of refraction.” But because the
composite material constructed by the UCSD physicists bends electromagnetic
radiation in the opposite direction, it is unique in possessing a “negative
index of refraction.”
“This is the first
demonstration of any material which has a negative index of refraction,” says
Smith. “Since no existing material has
this property, we needed to demonstrate the effect using a ‘metamaterial’—a
composite material fabricated from repeated elements, specifically engineered
to produce a desired electromagnetic behavior.”
The UCSD researchers have filed a patent application covering the
construction of the new composite material. Their study was supported by the
Defense Advanced Research Projects Agency, or DARPA, and the Air Force Office
for Science Research, or AFOSR, which are investigating potential applications.
The concept of metamaterials has been recently introduced as part
of a new DARPA initiative. Earlier this
year, researchers at Marconi Caswell in England and London’s Imperial College
demonstrated improvements to a magnetic resonance imaging system (MRI) using a
magnetic metamaterial based on a structure similar to that of the left-handed
structure developed by the UCSD team.
Intrigued by the possibility
of such a material, John Pendry, a physicist at Imperial College who laid the
groundwork for the UCSD development through his earlier publications, last fall
published a paper in the journal Physical
Review Letters, asserting that a material with a negative refractive index
could make a “perfect lens.” This is because such a lens would not be limited
by a diffraction limit, a condition that now prevents an ordinary lens from
focusing the light that enters its surface into a spot smaller than
approximately half a wavelength in diameter.
Although the composite
material constructed by the UCSD physicists cannot focus visible light, that
obstacle may one day be removed in future negative refractive index materials.
“We don’t believe that’s a fundamental limitation,” says Schultz. “We may be
able to find a way to go to visible light someday.”
The material used for the
experiment—which consists of a series of thin fiberglass sheets coated with
copper rings and wires, and arranged into squares like the interlocking inserts
in a case of wine—was painstakingly constructed by Shelby based on a computer-
assisted design.
While it behaves in a manner consistent with the laws of physics, the
composite 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.
As a result, it is part of a class of materials the UCSD
physicists refer to colloquially as “left-handed materials,” after a term
coined by Russian theorist V. G. Veselago, who predicted the possibility of
such materials in 1968, because they reverse this relationship as well as many
of the physical properties that govern the behavior of ordinary materials.
Another property the material is predicted to reverse 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 from a moving source 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 from an observer.