Visible Light Enters the Bizarro World

Wrong way. A glimpse at negative refraction in a pond.

N. Kevitiyagala/Science

They're still a ways off, but invisibility cloaks and microscopes with superresolution could now be a big step closer to reality thanks to a pair of results to be reported this week. For 8 years, physicists and engineers have tinkered with metamaterials, patterned arrays of bits of metal and insulator that bend and manipulate microwaves and shorter wavelength radiation in strange ways. Now, a team has made three-dimensional miniaturized metamaterials that work with near-infrared and visible light. That's a key step toward superlenses and cloaks for visible light, some say. Others say the claims are overblown.

Metamaterials put a kink in the way light usually passes from one medium into another. Suppose light from the setting sun shines on a pond. As light waves strike the surface, their direction will change so that they flow more directly down into the water. (See diagram.) Such "refraction" arises because the light travels more slowly in water than in air, giving water a higher "index of refraction." Still, the light continues to flow from west to east. Were water a "left-handed metamaterial," however, the light would undergo "negative refractions" and bend back toward the west. Refraction is the key to how ordinary lenses focus light, and in theory, negative refraction would allow a flat slab of metamaterial to function as a lens that could focus light infinitely tightly.

Physicists unveiled the first left-handed metamaterial for microwaves in 2000 (ScienceNOW, 23 March 2000). Looking a bit like a high-schooler's science-fair project, it was an assemblage of metallic rods and rings that interacted with and bent microwaves in strange ways. Since then, researchers have been pushing to shorter and shorter wavelengths, and with the new studies, the visible realm is within sight.

On Thursday online in Nature, Xiang Zhang, an applied physicist at the University of California, Berkeley, and colleagues will describe a metamaterial that works for near-infrared light and, unlike previous materials for such light, is three-dimensional. Zhang, Jason Valentine, and colleagues created a material that looks like a miniature waffle. They laid down 21 alternating layers of insulating magnesium fluoride and conducting silver on a quartz substrate and drilled holes in the stack using an ion beam. They then shaved off the stack's top at an angle to make a prism and showed that it bent light the "wrong" way compared with an ordinary prism.

In Science on Friday, the team will present a different three-dimensional metamaterial that bends visible red light in the desired way. In this case, Zhang, Jie Yao, and the team used a standard electrochemical technique to make a sample of aluminum oxide filled with a regular array of nanometer-sized holes. They then filled the holes with silver. When they shined red light onto the sample at an angle, it underwent negative refraction.

That might seem to clinch the case for metamaterials for visible light. But Henri Lezec, an electrical engineer at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, says "the claims are misleading and overhyped." Lezec, who last year demonstrated negative refraction of visible light in a two-dimensional "waveguide", argues that the infrared metamaterial isn't truly three-dimensional because it works for light coming from only a narrow range of directions. He notes that the metamaterial for visible light works only for light polarized in a specific direction, and all agree that that metamaterial does not have a key property--a negative index of refraction--although the infrared metamaterial does.

But that's nitpicking, says Vladimir Shalaev, a physicist at Purdue University in West Lafayette, Indiana. "What's wrong with [using] a particular polarization?" he says. "As a first step, it's not so bad." The real advance, Shalaev says, may be in introducing a new self-organizing technique to make the nanometer-scaled patterns in the materials.

Costas Soukoulis, a theorist at Iowa State University in Ames and the Department of Energy's Ames Laboratory, says that the Science paper in particular raises the possibility of making rudimentary superlenses that work in the visible spectrum: "This is a big step forward."

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