Invisibility used to be the stuff of comic books and Harry Potter novels. But this week, scientists from UC Berkeley have emerged with two new invisibility-producing "metamaterials," engineered substances that bend electromagnetic waves in ways they've never bent before. They call it "negative refraction." But you and I can just call it the first step towards invisible armor. We talked to one of the Berkeley scientists involved, and got the scoop.Controlling the way light rays bounce off of and move through objects is no easy feat, but that's exactly what Berkeley's metamaterials do. All naturally-occurring materials have a positive refractive index. As light waves travel from one medium to another, the difference in the refractive index between the two will cause the light wave to bend at a certain angle. Consider what happens when you stick a straw into a glass of water — the straw appears to bend or break as it enters the water. What you're seeing is the way light bends as it moves from the air (which has a refractive index of about 1) and water (which has a refractive index of about 1.33). The light is still propagating forward, but it's made a slight turn, and so your eyes see a bendy straw. S In the case of negative refraction, the light waves behave much more oddly, as you can see in the above image by UC Berkeley's Jason Valentine and Robert Lee. Valentine explained to me that in negative refraction, a light ray no longer appears to be propagating forward — when it bends, it bends backward. The energy flow of the wave still moves in its forward direction, but the electric and magnetic components of the light ray seem to be traveling in reverse. They've turned far more drastically than they would in the natural phenomenon of positive refraction. So instead of seeing a bendy straw, once the metamaterial is combined with other light-bending tech, you'd see a straw that seemed to disappear. In order to manipulate light at this level, you have to manipulate the structure of the material it's hitting at an extremely small scale. That's where metamaterials come in. Metamaterials negatively refract waves of visible light because they're woven out of materials smaller than the wavelengths of that light. If you think of a metamaterial as a piece of cloth, the "threads" in that make it up are somewhere between 400 and 700 nanometers in size. As fabrication techniques for such metamaterials have grown more and more advanced, this nanoscale structural manipulation has become possible, and UC Berkeley's team has used it to full advantage. According to a release about Valentine's study:
"What we have done is take two very different approaches to the challenge of creating bulk metamaterials that can exhibit negative refraction in optical frequencies," said Xiang Zhang, professor at UC Berkeley's Nanoscale Science and Engineering Center, funded by the National Science Foundation (NSF), and head of the research teams that developed the two new metamaterials. "Both bring us a major step closer to the development of practical applications for metamaterials."
A paper in the August 13 issue of Nature, co-authored by Valentine, Shuang Zhang, and Thomas Zentgraf (all members of Xiang Zhang's lab), explores one of these approaches. Valentine, Zhang, and Zentgraf layered conducting silver and non-conducting magnesium fluoride. Then, they cut tiny "fishnet" patterns into the material. The result is a metamaterial, pictured at the top of this page, that is capable of achieving a negative index of refraction at wavelengths as small as 1500 nanometers. The second approach, detailed in the August 15 issue of Science, appears in a paper co-authored by Jie Yao, Zhaowei Liu, and Yongmin Liu (also all members of Zhang's lab). What these researchers did was grow silver nanowires inside aluminum oxide, to create a bulk metamaterial that is more than 10 times larger than the wavelength of visible light. The structure of this metamaterial, however, is still on a nanoscale. Though the Science metamaterial doesn't technically have a negative index of refraction, the geometry of its structural components interacts with light in a way that still achieves the backward-bending phenomenon of negative refraction. And it does this with light rays that have wavelengths as short as 660 nanometers. The media is aflutter with ideas for possible applications of these new metamaterials, and they run the gamut from the visualization of individual molecules of DNA to the production of Harry Potter's invisibility cloak. Valentine cautions that cloaking devices are still in the future — in order to make things truly invisible, one would need to cover them with a large sheet of a metamaterial like these, and that's a fabrication challenge. In addition, though researchers have made a breakthrough in the way manufactured materials can control the bending of light rays, actual invisibility demands that each of the light waves around a given object are deflected in a certain way, creating a specific pattern of refraction that will hide that object. Still, these metamaterials are making my heart beat faster. It's hard to deny the excitement that comes with knowing we can build substances that move light in ways no existing material can — that as far as refraction goes, we've got a one-up on nature. Invisibility shields one step closer with new metamaterials that bend light backwards [UC Berkeley]