bends microwaves backwards
Technology Research News
Researchers from the University of California
at San Diego have fashioned a material that makes microwaves bend backwards.
The material uses a series of tiny strips and c-shaped pieces of copper
painted on the surfaces of a three-dimensional structure made from circuit
board and cut to resemble the cells that make up the inside of a wine
When the researchers put microwaves of a certain frequency through a prism-shaped
chunk of this material, the microwaves came out the other side bent far
enough downward to show that the material has a negative index of refraction.
The refractive index of a material is a measure of how much electromagnetic
waves slow when they enter it. The larger the index number, the more the
waves bend, and therefore change direction, when entering the material.
Air, like a vacuum, has a refractive index of one, and water has a refractive
index of about 1.3. Several feet of water slow, and thus bend, light discernibly,
making objects at the bottom of a swimming pool appear closer than they
really are. The refractive index of glass, about 1.5, has a lot to do
with how glass lenses focus lightwaves.
A negative index of refraction makes waves bend in a direction opposite
to the way they travel through other materials.
In the researchers' experiment, the microwaves entered the base of the
triangular prism and came out the hypotenuse, which is the longest side.
The normal is a line perpendicular to the hypotenuse that bisects the
space outside it. Materials that have a positive index of refraction reflect
rays that enter the base out from the hypotenuse to the right of the normal.
With the researchers' material, however "microwaves bend not only closer
to the normal instead of away from the normal, they cross over to the
other side of [the] normal. It's what we would call a negative angle rather
than a positive angle," said Sheldon Schultz, a physics professor at the
University of California at San Diego. "They come out on the other [side
of the normal, but] at the same angle to the normal, which makes it like
a sharp, left-handed turn," he said.
The researcher's material causes a negative index of refraction for only
a narrow range of microwaves, which are longer than visible light. But
it's easier to picture how this opposite bend works out using visual examples:
Lightwaves from a flashlight traveling through a slab of material that
has a negative index for light would reconvene into a single point on
the other side. A negative index of refraction would also make a concave
lens bend light inward toward a focal point like a convex lens normally
does, and make a convex lens bend light outward like a concave lens normally
A negative index of refraction literally means that the direction the
peak of the wave is traveling is opposite that of the energy of the wave.
When you drop a rock in a pond, for example, it will cause a pattern of
waves to spread out from that rock. If you look closely you'll see that
both the group of waves and the peaks of the individual waves are moving
In contrast, a group of waves bent the wrong way by a negative index of
refraction would radiate out from the source, but the individual peaks,
or crests would go the opposite way. "They are running towards where the
rock went in, but the energy is going out. So you see [the group of waves]
further and further away from where the rock went in, but running towards
were the rock went in -- that's what the negative sign means," said Graham
Dewar, an associate professor of physics at the University of North Dakota.
Although somewhat counterintuitive, a negative index of refraction doesn't
break any laws of physics because the math works out, said Schultz. In
fact Russian physicist V. G. Veselago pointed this out in a little-known
paper published in 1968, Schultz said.
All electromagnetic waves harbor both electric and magnetic fields. In
order to have a negative index of refraction, a material must have both
a negative electrical field, or permittivity, and a negative magnetic
field, or permeability.
A material's index of refraction is the square of its permittivity times
its permeability. The counterintuitive part is, because a negative number
times a negative number is a positive number, it seems like the index
of refraction is destined to remain positive. "When you go to take that
square root, if you are a little sloppy you think of it also as positive.
But because the negative of the square root can be positive or negative,"
it is mathematically possible to get a negative index of refraction, said
In making the negative index material, the researchers tapped the ideas
of John Pendry, a professor of physics at Imperial College in England
who worked out in the past few years that copper strips placed at certain
intervals would allow for negative permittivity, and c-shaped resonators,
The researchers combined Pendry's two forms by painting them on opposite
sides of each quarter- inch section of their material. This made a material
that was negative in both fields, giving it a negative index of refraction.
The material only affects the magnetic and electric fields of microwaves
of a certain frequency, and only when they pass through a chunk of the
material containing several sections. This makes the material a meta material,
which has composite properties that are not totally shared by the individual
units, said Schultz.
The material could eventually be used in electromagnetic devices like
antennas, filters and lenses, according to the researchers. "I believe
there will be several applications in wireless communication, but as to
time scale for implementation I hesitate to answer. In less than a year
we will have completed enough analysis" to know more, said Schultz.
It's a very sensible study that proves something true that was not widely
known, said Dewar. "There were a lot of statements that were floating
around that [a negative index of refraction] just doesn't happen, but
they're false," he said.
One challenge in using negative index materials, however, is they have
a narrow range, Dewar said. "There's a possibility of making your optics
better using [a negative index of refraction] but there's another limitation.
[These materials] are very, very frequency dependent and only work over
a [small] frequency range," he said, pointing out that the researchers'
material produces a negative index of refraction only for microwaves in
a range on the order of a few gigahertz.
One area where negative index of refraction materials may have potential,
however, is in nanostructures, Dewar said. "Nanostructures... might be
able to realize this negative index of refraction at frequencies much
higher than the microwave frequencies that Schultz used because once you
get smaller you can scale up the frequencies," he said.
Although it's theoretically possible to make materials that have a negative
index of refraction for infrared light, it is a difficult proposition
to do so for the visible spectrum, said Dewar. According to Schultz, the
researchers, however, aren't ruling out the possibility.
Schultz's research colleagues were Richard A. Shelby and David R. Smith
of the University of California at San Diego. They published the research
in the April 6, 2001 issue of the journal Science. The research was funded
by the Defense Advanced Research Project Agency (DARPA), and the Air Force
Office of Scientific Research.
TRN Categories: Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "Experimental Verification
of a Negative Index of Refraction," Science, April 6, 2001.
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