The man who built the world’s first invisibility cloak is back and this time he’s got an even better idea.
His first design was a triumph for headline writers and Harry Potter fans alike, although most glossed over the fact that this first cloak worked only for microwave-sensitive eyes and even then only at a single specific frequency. Oh, and only in two dimensions. And it wasn’t really a cloak at all, more of an invisibility canister.
Nevertheless, nothing should be taken away from the technical achievements of David Smith and colleagues at Duke University in North Carolina. They’ve done a job almost as spectacular as their PR team.
The new idea gets around one of the most pressing problems associated with invisibility cloaks which is that they are impossible to construct well. Invisibility cloaks work by distorting the permeability and permittivity of the cloaking material in way that forces light to bend around an internal cavity. This makes the cavity invisible to an observer.
But the technique requires the permeability and permittivity to take infinite values at certain points, particularly on the boundary between the cavity and and the cloaking material . And this just isn’t possible.
Various ideas have been proposed to get around this problem but all have their own weaknesses.
So Smith and colleagues say they might as well accept that an invisibility cloak cannot be perfect and use it to their advantage. Instead of attempting to hide the internal cavity completely or crushing it to a point as others have done, the new idea is to make it appear as a single line. Such an invisibility cloak wouldn’t hide an object entirely but instead make it look like a thin line, like a defect in the structure of the cloak.
That’s clever because it dramatically relaxes the constraints placed on the types of metamaterials you can use for cloaking. Smith and his buddies say that such a cloak would be “very easy to realize” using known techniques.
Smith is known for publishing theoretical predictions just ahead of the practical realisation.
So if his past form is anything to go by, we can expect to see a working invisibility cloak that employs this technique in the coming weeks or months.
Ref: arxiv.org/abs/0809.2317/: Invisible Cloak With Easily-Realizable Metamaterials