|An array of wire loops and lines bends microwaves in novel ways. Such composites could confirm theories of physics that seemed heretical just a few years ago.|
Photograph courtesy of David R. Smith/UCSD
"Every day you play with the light of the universe," wrote Chilean poet Pablo Neruda. Physicist David R. Smith of the University of California at San Diego takes those words to heart. He spends his days contemplating how to bend light in ways that reverse the normal patterns of refraction. And his are more than theoretical musings. Smith and a handful of like-minded researchers are now building mirror-image materials that could ultimately result in practical applications ranging from better cell-phone antennas to DVDs that could cram 100 movies onto a single disc. These playful manipulations of light were anticipated by the long-neglected ideas of Soviet physicist Victor Veselago of the Russian Academy of Sciences in Moscow. In 1964 Veselago theorized that the laws of physics allowed for the creation of a material that he described as "left-handed" because of the way it would affect light or other radiation passing through it. In conventional materials a ray of light moving forward has an associated electric field that points left and a magnetic field that points up. You can picture this arrangement by pointing the thumb, forefinger, and middle finger of your right hand in mutually perpendicular directions. In Veselago's bizarre material, these orientations would be mirror reversed, so they would follow the pattern of the fingers on the left hand instead.
This seemingly simple change would lead to some very peculiar consequences. Perhaps the most surprising one is a negative index of refraction, the measure of how a given material bends light. Glass, water, and every other known substance have positive indices of refraction. That is why a swimming pool looks shallower than it really is and why sunlight comes to a point when it passes through a magnifying glass. A left-handed magnifying glass, by contrast, would disperse light, while a concave lens or even a flat piece of left-handed material would bring light to a focus. All most strange. But Veselago had no notion of how to make such a material, so his work languished in obscurity.
Three decades later John Pendry of Imperial College in London recognized that even though left-handed materials don't exist in nature, it might be possible to construct composite structures, or meta-materials, that have left-handed traits. The building blocks of such meta-materials would be small metallic structures that collectively respond to electricity and magnetism in novel ways. For instance, the electric charges within a substance normally try to align themselves with an applied electric field. But in an array of thin, conductive wires, the surface electrons are sluggish and respond out of step so that the charges oppose rather than align with the electric field. In 1999 Pendry and his colleagues at Imperial College and Marconi Caswell Limited, in Northamptonshire, England, performed calculations showing that a split ring of metala hoop with a small gap in it, like the letter C
could exhibit a similarly reversed response to magnetism.
Two papers describing this work caught the eye of Smith and Sheldon Schultz, another physicist at the University of California at San Diego, who set out to create a full-blown left-handed material. They found that a double split ring, one metallic hoop nestled inside the other, worked even better than a single ring. After some experimentation, Smith, Schultz, and graduate student Richard Shelby imprinted an array of split rings in copper film on one side of a standard fiberglass printed circuit board. Then they added an array of parallel metal wires on the other side. Together, the rings and the wires inverted both the electrical and the magnetic properties of the light. Finally, the researchers slotted the boards at right angles, like the cardboard compartments in a crate of wine, to create a left-handed prism.
|A pane of normal, right-handed material (top) slightly disperses light. Left-handed material (bottom) brings the rays to a focus. A flat slab of such material could, in principle, act as a super-microscope.|
Graphic by Matt Zang
The next step was to test whether the material would bend light waves the way Veselago imagined. The split-ring design works only on wavelengths longer than the size of the rings, which are about one-fifth of an inch wide. That is far longer than visible light; it is the wavelength of microwaves. So the researchers passed a microwave beam through their meta-material to explore its properties. Last spring they reported their results: The bending of the microwaves indicated a negative index of refraction as large as -2.7.
The result came soon after the Defense Advanced Research Projects Agency decided to fund research to develop meta-materials "with superior microwave and/or optical properties for communication, radar, and wireless power transfer application." The left-handed materials created by the San Diego research team fit the bill. "Meta-materials with a negative index of refraction may reduce the cost and improve the performance of many devices used in wireless communications," Smith explains. He and his colleagues are also working with Lucent Technologies to explore ways to make antennas that can handle high data-transmission rates. Because of their inverted properties, left-handed materials might make it possible to place multiple antennas close together while generating only minimal interference. "We have not even begun to tap the applications at microwave frequencies, let alone what may lie ahead at optical frequencies," Smith says.
Pendry already has a vision of what a visible-light left-handed material might do. He has shown that it could act as the perfect lens. Conventional lenses cannot reveal any detail smaller than the wavelength of light they are focusing, so they hit a wall of resolution at several hundred nanometers, about 1/100,000 inch. This limit applies to all light that is propagating through space. But strange as it seems, not all light propagates. There is also a stationary electromagnetic glow, known as the near field, surrounding all light-emitting or light-reflecting objects. The near field contains the fine detail that is normally lost from the image, but it is so narrowjust a few tens of nanometersthat it is inaccessible to ordinary optics. In 2000 Pendry recognized that a flat lens composed of left-handed material could focus and amplify the near-field radiation and so yield an image whose resolution is far smaller than one wavelength. Last year's experiment by the San Diego research team affirmed that such a lens really could exist, for microwaves at least.
What engineers really crave, however, is a perfect lens that bends visible light. Then they could etch much smaller circuits onto computer chips, or burn many more spots of data onto a single DVD, or study now-invisible physical processes that occur on the surfaces of materials. To meet such a challenge would require shrinking the components of the left-handed material by a factor of 10,000 or more. Smith explains that his research team's design won't work for wavelengths shorter than infrared because the electrical properties of the metals change at smaller scales. "There is as yet no optical analogue for the material we developed," he says.
Photonic crystalsa class of composite materials whose basic building blocks are microscopic spheresmight be able to provide a negative refractive index for visible light. "But this effective negative refraction doesn't appear to have the same remarkable properties with respect to the near field, so the prospects for optical versions aren't clear," Smith says. Pendry is moving in the other direction, developing a rolled-up meta-material that bends long-wavelength radio waves. He hopes to use it to focus the waves that are used to form images in MRI scanners so that doctors can zero in on one small region of the body.
Even as he contemplates practical applications, Pendry is still amazed that nature allows the left-handed world to exist at all: "People have been surprised by this behavior, and that includes myself. Sometimes I feel like a heretic." x
David Smith and his colleagues at the University of California at San Diego explain their work on left-handed materials: www-physics.ucsd.edu/lhmedia