PHYSICS New Material Reverses Snell

Snell's Law Reversal - Negative Index of Refraction

Updated: 07-24-06

PHYSICS - New Material Reverses Snell's Law

snell
Image: Richard A. Shelby, U.C.S.D.

Okay, you may not know Snell's law offhand, but you experience it every day. Whenever you put on a pair of glasses, or see light bend through a glass of water or a prism, this rule is in action. In short, Snell's law governs the angle by which electromagnetic radiation such as light refracts, or changes direction, as it passes from one material to another and slows down. The more the radiation slows, the more it bends and the higher the material's so-called index of refraction, described by Snell.

Until now, all known materials had a positive index of refraction. But scientists from the University of California at San Diego described in Friday's issue of Science a strange composite that has a negative index, essentially reversing Snell's law. This new mix of fiberglass and copper rings and wires (see image) is far more than a curiosity, the researchers say. It may very well lead to novel electromagnetic devices and even perfect lenses, unhindered by diffraction limits and thus capable of focusing light in unforeseen ways.

Sheldon Schultz and his colleagues produced the class of composites last year, predicting at the time that it would defy a number of ordinary properties, including the Doppler effect. Their recent demonstration is only the first of what they hope will be several to reveal the composites' unorthodox behavior. In this case, they showed that microwaves—at the same frequency as those used in police radar guns—emerged from the material in the exact opposite direction from that predicted by Snell's law. Next they hope to extend the material's powers to focusing visible light.

"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," Schultz says. "There's no way you can do that with just a flat sheet of ordinary material." The Defense Advanced Research Projects Agency (DARPA) and the Air Force Office for Science Research (AFOSR), which supported the research, are studying possible applications, and the U.C.S.D. researchers have filed a patent application. —Kristin Leutwyler

Reversing And Accelerating The Speed Of Light

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by Staff Writers
Ames IA (SPX) Jul 25, 2006
Physicist Costas Soukoulis and his research group at the U.S. Department of Energy's Ames Laboratory on the Iowa State University campus are having the time of their lives making light travel backwards at negative speeds that appear faster than the speed of light.

That, folks, is a mind-boggling 186,000 miles per second - the speed at which electromagnetic waves can move in a vacuum. And making light seem to move faster than that and in reverse is what Soukoulis, who is also an ISU Distinguished Professor of Liberal Arts and Sciences, said is "like rewriting electromagnetism." He predicted, "Snell's law on the refraction of light is going to be different; a number of other laws will be different."

However, neither Soukoulis nor any other scientist involved in efforts to manipulate the direction and speed of light can do so with naturally occurring materials. The endeavor requires exotic, artificially created materials. Known as metamaterials, these substances can be manipulated to respond to electromagnetic waves in ways that natural materials cannot.

Natural materials refract light, or electromagnetic radiation, to the right of the incident beam at different angles and speeds. However, metamaterials, also called left-handed materials, make it possible to refract light at a negative angle, so it emerges on the left side of the incident beam.

This backward-bending characteristic of metamaterials allows enhanced resolution in optical lenses, which could potentially lead to the development of a flat superlens with the power to see inside a human cell and diagnose disease in a baby still in the womb.

The challenge that Soukoulis and other scientists face who work with metamaterials is to fabricate them so that they refract light negatively at ever smaller wavelengths, with the ultimate goal of making a metamaterial that refracts light at visible wavelengths and achieving the much-sought-after superlens.

Admittedly, that goal is a ways off. To date, existing metamaterials operate in the microwave or far infrared regions of the electromagnetic spectrum. The near infrared region of the spectrum still lies between the microwave and visible regions, and the wavelengths become ever shorter moving along the electromagnetic spectrum to visible light.

Correspondingly, to negatively refract light at these shorter wavelengths requires fabricating metamaterials at extremely small length scales - a tricky feat.

However, recent research by Soukoulis and his co-workers from the University of Karlsruhe, Germany, published in the May 12, 2006, issue of Science demonstrates they have done just that. "We have fabricated for the first time a metamaterial that has a negative index of refraction at 1.5 micrometers," said Soukoulis.

"This is the smallest wavelength obtained so far." Small, indeed; these wavelengths are microscopic and can be used in telecommunications. Soukoulis' success moves metamaterials into the near infrared region of the electromagnetic spectrum - very close to visible light, superior resolution and a wealth of potential applications!

In addition, Soukoulis and his University of Karlsruhe colleagues have also shown that both the velocity of the individual wavelengths, called phase velocity, and the velocity of the wave packets, called group velocity, are both negative, which Soukoulis said accounts for the ability of negatively refracted light to seemingly defy Einstein's theory of relativity and move backwards faster than the speed of light.

Elaborating, Soukoulis said, "When we have a metamaterial with a negative index of refraction at 1.5 micrometers that can disperse, or separate a wave into spectral components with different wavelengths, we can tune our lasers to play a lot of games with light. We can have a wavepacket hit a slab of negative index material, appear on the right-hand side of the material and begin to flow backward before the original pulse enters the negative index medium."

Continuing, he explained that the pulse flowing backward also releases a forward pulse out the end of the medium, a situation that causes the pulse entering the front of the material appear to move out the back almost instantly.

"In this way, one can argue that that the wave packet travels with velocities much higher than the velocities of light," said Soukoulis. "This is due to the dispersion of the negative index of refraction; there is nothing wrong with Einstein's theory of relativity." (These effects are clearly seen in the simulations that accompany this press release. Go to: Light Movies)

The Basic Energy Sciences Office of the DOE's Office of Science funds Ames Laboratory's research on metamaterials. Ames Laboratory is operated for the Department of Energy by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.