Some years ago scientists managed to build a rudimentary invisibility cloak, which was an impressive device but it had some important limitations, not least of which was that it worked only for a single frequency of microwaves.
One of the biggest questions that physicists have puzzled over since then is whether it is possible to build cloaking device that works over the range of frequencies visible to the human eye.
Last year, a couple of groups announced a solution to this problem in the form of ‘carpet cloaks’ that lie over an object, hiding its presence over a range of optical frequencies.
Again, these were impressive feats but with some limitations. These cloaks are made of finely carved silicon microstructures and so were expensive to build. And they can only hide objects up to a few micrometres in size, not much bigger than the wavelength of light itself.
Recently, Baile Zhang at the Massachusetts Institute of Technology in Cambridge at a couple of buddies have done significantly better. They’ve built a carpet cloak capable of hiding objects in the millimetre range over a broad range of visible frequencies from red to blue. Indeed, this is still far from having a carpet to hide Harry Potter under, but impressive nonetheless, if only because they’ve built this cloak out of calcite, an ordinary and relatively cheap optical material, using conventional optical lens fabrication techniques. This makes the cloak cheap and easy to build.
Carpet cloaks sit on a surface covering the object to be hidden. Their trick is to make it look as if light is reflecting off this surface, thereby hiding any object that they cover. So far, this has only been done using artificially modified structures that steers light in a specially engineered ways. This so-called metamaterial is a kind of wonder substance that is the focus of great attention right now.
However, Zhang and co realised that there are non-man made materials that can do the same thing. Calcite is one of them. It is unusual because its optical properties depend on the direction that light passes through it.
By carefully exploiting this property, they’ve been able to create a block of calcite (actually two blocks of calcite) that acts like a carpet cloak. They’ve even demonstrated it by hiding a wedge of steel 38mm long and 2 mm high. Zhang and co say that this is the first time that a visible object has ever been cloaked.
Their cloak has its limitations, of course. The main one is that it only works in a single 2D plane, so the object is hidden only to those looking from a certain direction. Another is that it works only with polarised light. But that’s not as limiting as it may seem at first sight. Water tends to polarise light so it seems reasonable to think that the cloak ought to work well underwater.
It wasn’t so long ago that some physicists were saying that optical invisibility cloaks would always be impossible (because metamaterials tend to absorb visible light faster than they can transmit it).
That’s turned out to be of little concern and invisibility cloaks just get better and better. In fact, it’s hard to think of a technology that has advanced so far, so quickly. To be continued.