"Invisibility cloaks" have been in development for years, but keep hitting a major roadblock — the materials used absorb too much light. New research may have found the trick to get around that, and get us our cloaking devices.

The premise of cloaking devices hinge on metamaterials — materials which are engineered to have abilities not found in nature. This stuff of superheroic legend can be created for any number of purposes — and one of the most prominent is to have a refraction index of below one, or ideally below zero. In other words, light doesn't refract when it hits them, which would be invaluable for creating ultra-sensitive lenses, microscopes powerful enough to see DNA, better solar panels, and usable invisibility.

The problem is that the materials used in the construction of metamaterials — notably gold and silver — absorb too much light, making them remarkably inefficient. That's where the research of Shumin Xiao of Purdue University comes in. In research published this week in Nature, the scientists discovered a way to make the metamaterials massively more efficient.

These metamaterial cloakers are made up of repeating layers of silver and aluminum oxide, in a fishnet-like film with holes of 100nm. Where most previous work involved adding another layer of material to boost the light — which wasn't efficient enough to counter the loss — the Purdue researchers went a different route. They etched away at the aluminum oxide between the silver layers, and filled it with a special dye that can amplify light when used at a sufficiently small scale. Putting this dye inside the layers rather than above created a 50x increase in its efficiency.

Vladimir M. Shalaev, Purdue's Robert and Anne Burnett Professor of Electrical and Computer Engineering, said:

"What's really important is that the absorption coefficient can be as small as only one-millionth of what it was before using our approach. We can even have amplification of light instead of its absorption. Here, for the first time, we showed that metamaterials can have a negative refractive index and amplify light."

However, manufacture of this new metamaterial is hardly an easy task. The maximum amount of aluminum oxide has to be removed, but enough has to remain behind to keep structural stability; and the dye layer has to be precisely placed, and not more than 50nm thick.


While this research is funded by the U.S. Army Research Office and the National Science Foundation, given the difficulty manufacturing it, it might be a while yet before invisible soldiers stalk the battlefields.

[via Purdue Newsroom]