io9: mad materials science

Secrets of the Metamaterials that Will Make You Invisible

Nivair H. Gabriel — Tue, 12 Aug 2008 14:20:00 PDT

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.

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]

Your Floor Can Glow in the Dark with Luminescent Gravel

Annalee Newitz — Tue, 18 Mar 2008 17:02:20 PDT

Ever wanted to create luminescent designs or safety arrows in your floor, but weren't too crazy about those crappy, glow-in-the-dark stickers? Now a company in the Netherlands called Hidden Safety is marketing luminescent gravel, which can be mixed into concrete floors to create beautiful patterns or just point people the right direction in the dark. The gravel looks ordinary in light, but it is actually absorbing photons (hence, the material is often called photoluminescent). When darkness falls, photons zoom out of the gravel and create a glow.

You can use different colors to create the glow, with many different patterns.

Some prefer to use them as emergency arrows for people in industrial settings or on cruise ships.

Luminescent Gravel [Transmaterial]

Smart Paint Creates Moving Logos on Your Pop Bottles

Annalee Newitz — Tue, 04 Mar 2008 07:00:21 PST

One squirt of smart paint could create a small moving picture on the next bottle of pop you buy. This paint, which is being developed at the University of Warwick in the UK, essentially smears an electrical power system onto any surface: bottle, computer, even your hands. The electrified surface could then display a moving image; or it could provide camouflage, changing color to match its environment. Or it could become a tracking device, perfect for a surveillance society.

According to Warwick researcher Gordon Smith:

This technology opens up a wealth of possibilities, plastic drink bottles could have moving displays created as an integral part of the bottle - or instead of tracking products by hiding RFID tags in them the whole product or a major plastic component of it could effectively be turned into a giant impossible to remove tag.

Smart paint has long been a dream of the U.S. military as well. It funded a research project at the New Jersey Institute of Technology in 2002 to create self-healing paint that could repair itself and prevent rust on tanks. Image via Painted Soda Bottle Collectors Association.

Smart Paint [Printed Electronics World]

Translucent Concrete Lets The Light Shine In

Kevin Kelly — Mon, 07 Jan 2008 12:00:24 PST

Get ready for windows that never break with translucent concrete. A German design firm has created load-bearing concrete containing optical fibers, allowing light and color to pass through to the other side (the shadowy hand is what you can see through the concrete in direct light). The result is that you can live inside a sunlit dome and still be protected when the space invaders come and drop those dangerous light thingies on your head. Click through for more transluscent concrete, and some transparent aluminum too.

Ha! It's Scotty's formula for transparent aluminum from Star Trek. Amazing what those Mac Classics could do.

What's particularly impressive about translucent concrete is that the optical fibers only make up 4% of the mixture, which is what allows (they claim) the concrete to retain the same "technical data" as normal concrete. In layman's terms we're assuming this means "Pretty light shine through, house not fall down." Based on the photos of the concrete in action, and if this load-bearing claim holds up (get it?) then we imagine that this would get put to immediate use in the construction of formerly boring government buildings everywhere.

In fact, now is a good time to think about rebuilding the io9 bunker entirely out of this stuff. We can't wait on Star Trek tech anymore, unless someone from the future traveled back in time and told someone at this firm how to make this stuff. Then the whole time-travel paradox of Star Trek IV will open up like a can of worms and consume us as we sit in our see-through outbuildings.