Invisibility Cloak comes Closer to Reality!
December 17, 2010 Leave a comment
There has been no shortage of column inches devoted to invisibility cloaks since engineers built the first one back in 2006. This 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 similar devices that work 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.
It wasn’t long ago that some physicists said that optical invisibility cloaks would be impossible to build. Earlier, two teams claimed to have built cloaks that work over a wide range of optical wavelengths, and the extraordinary thing is that both designs are almost identical. Invisibility cloaks work by steering light around an object, fooling an observer who sees nothing but the background view. But while this works well for microwaves, it is not a straightforward matter to shrink these cloaks to a size that works at optical wavelengths.
They have both built cloaks that are essentially mirrors with a tiny bump in which an object can hide. The cloaking occurs because the mirrors look entirely flat. The bump is hidden by a pattern of tiny silicon nanopillars on the mirror surface that steers reflected light in a way that makes any bump look flat. So anything can be hidden beneath the bump without an observer realizing that it is there, like hiding a small object under a thick carpet.
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 upto a few micrometres in size, not much bigger than the wavelength of light itself. Now, Baile Zhang at the Massachusetts Institute of Technology in Cambridge at a couple of buddies have done significantly better. They’ve built acarpet cloak capable of hiding objects in the millimetre range over a broad range of visible frequencies from red to blue. More impressive than this is that 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.Until now, 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 naturally occurring 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 38 mm long and 2 mm high. Zhang and co say that this is the first time that a visible object has ever been cloaked. That’s impressive.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.
[Ref: arxiv.org/abs/1012.2238: Macroscopic Invisible Cloak for Visible Light]