Material bends microwaves backwards

By Kimberly Patch, 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 box.

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 does.

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 outward.

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 Schultz.

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, negative permeability.

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.

Timeline:   Unknown
Funding:   Government
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.


April 11, 2001

Page One

Glass mix sharpens holograms

Material bends microwaves backwards

Shaky chip makes for bug-eyed bots

Cold plastic gives electrons free ride

Holographic technique stresses interference


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