Normal 0 false false false MicrosoftInternetExplorer4 At this rate, Harry Potter skeptics may soon be left with no place to hide. Two new materials that bend light backward suggest that invisibility cloaks like Harry’s could some day become feasible.
The two materials, one described in a paper to appear in Science and the other reported online by Nature on August 11, take different approaches toward the same goal, says Xiang Zhang of the University of California, Berkeley, senior author of both papers. That goal is to bend light in the opposite direction than it would when entering an ordinary material.
For example, light bends at the surface of a pond, making fish look larger and closer than they really are. If water bent light the opposite way, fish would instead appear to float above the surface.
The possibility of reverse bending has been known for decades, but it wasn’t until 2000 that scientists demonstrated materials that could do the trick. The first demonstrations worked for microwave radiation, which has longer wavelengths than those of light. And last year, the first materials began to appear that could bend visible light backward.
But until now, those materials had many drawbacks. For example, they let very little light through, or they worked only for a particular polarization of light. The Berkeley materials, while not completely transparent, remove some of those limitations.
There still is a long way to go before real cloaking becomes reality, says theoretical physicist Ulf Leonhardt of the University of St. Andrews in Scotland, “but the Berkeley team made an important step.”
In the Nature paper, the authors describe a film made of about 20 alternating layers, each just tens of nanometers thick, of silver and magnesium fluoride. Holes about 800 nanometers apart penetrate all the layers, creating a fishnet appearance.
As light rays fall on the material, the light’s electric fields induce a small displacement in the electrons of both types of layers. The electrons’ displacement itself produces electric fields, which add up to those of light and help turn the waves at an opposite angle.
Previous attempts — including a version of the fishnet made of a single aluminum oxide layer sandwiched between silver layers — typically exploited the phenomenon of resonance, explains Jason Valentine, a coauthor of the Nature paper. That meant that the materials were optimized to absorb energy from light and turn it into currents. But too much absorption made the materials virtually opaque, Valentine says.
In the new fishnet structure, the presence of multiple layers removes the resonance. So more sandwiches stacked up are transparent, while the single sandwich isn’t. “It is quite counterintuitive,” Valentine says.
The fishnet works in the infrared spectrum, but the same principle should work with visible light, if the holes are cut smaller, Valentine says. “It’s just a question of fabrication.”
In the Science study, researchers created a new kind of material by embedding 60-nanometer-thick silver wires in an aluminum-oxide sheet just one-hundredth of a millimeter thick. The wires were all aligned with one another and cut through the sheet at a perpendicular angle.
Light displaces electrons in the silver nanowires and bends backward, while the aluminum oxide keeps the material transparent — to a degree. The 10-micrometer-thick sheets still appear almost black to the eye, says Jie Yao, a coauthor of the Science paper. But the material does work for visible light, Yao’s team reports.
Neither material is yet ready for full-fledged cloaking status. For example, both bend light at different angles depending on its wavelength, while a real invisibility cloak should bend light equally across the spectrum.
In principle, a material that bends light backward could be fashioned into a spherical shield that would hide what’s inside by making light flow around it, researchers have suggested (Science News 7/15/06, p. 42).
Such materials would have other applications beyond cloaking. For example, they could be used in microscopes that don’t suffer from the same limitations of ordinary optics, which cannot image details smaller than one-fifth of a micrometer. For microscopy, it would be sufficient to use one wavelength at a time, Valentine points out, so it would not be necessary to bend all wavelengths by the same angle.
“Being able to bend light in unusual ways is important for applications that almost resemble magic,” Leonhardt says.
