In the cat and mouse game of camouflage, the extraordinary development of invisibility cloaks has surely given the mouse the upper hand in recent years. Not only have theorists dramatically developed the ideas behind these devices, but others have built and tested the cloaks themselves. The world of camouflage will never look the same.
Today, the cats fight back by revealing how to detect the presence of an invisibility cloak and to discover the object it hides.
Invisibility cloaks hide objects by steering light around them so that the objects cannot be seen by an observer. The trick is to build a material that steers light in this way.
That turns out to be be extremely hard, partly because the optical properties of these materials has to change from one point to another in a way that achieves this steering, a property known as optical anisotropy. Because of this, the first invisibility cloaks had to be handbuilt and even then managed only to cloak a flat object in two dimensions at a single wavelength of microwave light. Nobody could really imagine how this approach could be made to work on the much smaller scale required for visible light nor over a range of frequencies.
Then a couple of years ago, John Pendry, the theoretical physicist at Imperial College London who has been the intellectual driving force in this field, came up with another approach: the carpet cloak.
The idea was that a layer of dielectric material on a surface could bend light in a way that made it look as if the light were reflecting off the original surface. In other words, this extra layer would be invisible, so anything contained within it would be invisible too.
This technique works for visible light at a wide range of frequencies. What’s more, said Pendry, materials that could do this could be entirely uniform–isotropic rather than anisotropic–and so would be much easier to make.
With this one idea, the dream of optical invisibility cloaks came within reach and on this blog, we followed the way this idea evolved from theory to practical devices in just a few months.
Since then, Baile Zhang and buddies at the Massachusetts Institute of Technology in Cambridge, have been busy looking for the weak point in this idea and now think they’ve found it. Today, they point out that carpet cloaks have a flaw that makes the objects within them detectable.
The problem, they say, is that isotropic cloaks cannot work perfectly. Here’s why. Light can be thought of as a series of wavefronts each with a certain amount of energy. Ordinarily, the direction of energy propagation is at right angles to these wavefronts.
However, in an invisibility cloak, this perpendicular relationship becomes distorted as the light waves are steered. That’s what an anisotropic material does. But an isotropic material cannot do this–the energy always propagates at right angles to the wavefronts. This limitation means that isotropic materials cannot hide objects in the way Pendry suggests.
Zhang and co go on to prove their assertion by tracing a ray that passes through the kind of isotropic carpet cloak that Pendry suggested. What they’ve discovered will shock carpet cloakers all over the world.
According to Zhang and buddies, carpet cloaks don’t hide objects, they merely shift them to one side by an amount that is just a bit less than they are high. Crucially the effect depends on the angle at which you are looking. So when illuminated at an angle of 45 degrees, an object 0.2 units tall appears laterally shifted by 0.15 units.
If Zhang and co are correct, this could be a substantial blow for isotropic carpet cloaking. It means that the carpet cloaking effect has a limited angle of view.
Look from directly above and an object can be perfectly hidden, you don’t need a carpet cloak for that, just conventional camouflage. The question is how far from the perpendicular can you go before an isotropic optical carpet cloak gives you away.
So in the invisibility wars, this battle goes to the cats. Of course, the mice will be back with a new and improved carpet cloak that gets around this problem. One obvious way out is to make the carpet anisotropic, in other words to vary its structure so that it properly steers light around the object.
Seconds out, round two.