Invisibility cloak becomes more than a possibility

Harry Potter fans do not need to be told how useful a real-life invisibility cloak could be. Now scientists around the world are getting closer to making this fictional fabric a reality.

The first working invisibility cloak, built in 2006, routed microwaves around a copper cylinder so that they emerged as if nothing was there. The cloak was made from metamaterials, substances whose properties result from their structure, not their composition.

For invisibility cloaks, the metamaterial consists of artificially structured materials that incorporate components much smaller than the wavelength of light and interact with light in a manner that does not exist in nature.

This cloak only worked in two dimensions and at a particular microwave frequency. To make an object literally vanish, a cloak would have to interact with all of the wavelengths that make light.

A study published earlier this year (2010) in Science showed that it is possible for a cloak to render an object invisible in three dimensions.

Researchers created a metamaterial cloak from a polymer crystal made of tiny rods. Altering the thickness of the rods changes the refractive index of the material. By tailoring the refractive index, the object under the cloak becomes invisible to a wide range of wavelengths slightly longer than visible light.

Although the study only cloaked a bump one micrometre high, in theory there is no limit to the size of object that could be hidden. The rods in this cloak were 200 nanometres, but they would have to be as small as 10 nanometers to hide an object from visible light.

More prosaic — but arguable more valuable — uses for invisibility cloaks include protecting buildings from earthquakes and protecting coastlines and offshore platforms from tsunamis. A number of large, plastic concentric rings fixed to the earth compresses surface earthquake seismic waves into tiny fluctuations in pressure and density that travel along the fastest path available.

By tuning the cloak’s properties, the path can be made into an arc that directs surface waves away from an area inside the cloak. Unlike some of the optical invisibility cloak studies, this cloak is “broadband”, meaning that it can divert waves across a range of frequencies. This is achieved by tuning different rings of the cloak to incoming waves of different frequencies. Waves pass largely unaffected through rings not tuned to their frequency.

These cloaks could be built into the foundations of new buildings, researchers suggest. A similar principle could be used to limit tsunami damage.