Now you see me

Ever wished you could slip out of sight? Invisibility cloaks, for a long time the staple of magic and sci-fi, are inching closer towards reality and unsurprisingly, getting a lot of attention for it.

Proposals for precisely how to build yourself a cloak vary, but almost all the suggestions currently on the table utilise the possibilities of metamaterial artificial materials which have been designed to have properties not usually found in nature. In particular, it’s possible to engineer the surface in such a way that it possesses some very odd optical properties. After all, the optical properties of an object are simply down to the way in which light bounces off the atoms which make up its surface.

Up until now, we’ve had to more or less take an object’s optical properties as given. For example, if you wanted a different colour of car, then your best option was to cover it with something possessing the desired surface structure and absorption properties (or in other words, paint it!). But modern technology is now at a point where these modifications can be made directly, allowing us to change the colour of substances such as gold, simply by changing the structure of the surface at an atomic level.

But the optical possibilities don’t stop there. For some time now, scientists have been investigating metamaterials whose atomic-level structure bends light as it passes through them in the opposite direction to natural substances such as glass or water. Using these materials, it could be possible to build ‘superlenses’, which may be used to ‘cancel out’ the light reflected from an object. The idea is to put the metamaterial ‘cloak’ very near to the object we want to hide. The light scattering off the object could interact with the surface of the cloak, causing it to reflect back light waves of a similar frequency, which would then interfere with and destroy those which bounced directly off the object, effectively rendering the object invisible.

An alternative approach is the idea of a ‘carpet cloak’, first suggested by Professor Sir John Pendry of Imperial College London in 2006. This utilises a different metamaterial oddity – their capacity to exhibit different, and finely controlled, refractive indices in different parts (i.e. different areas of the metamaterial bend the light by different amounts). In principle, it should be possible to place such a cloak around an object to make light flow around it undisturbed.

In spite of the significant technical hurdles for such a project, progress in this area has been made. A breakthrough came last year, when American researchers succeeded in creating a tiny, two-dimensional cloak (albeit for near-infrared, rather than visible light). The scientists took a sheet of silicon, and carefully drilled it with tiny holes. As each hole was less than the light’s wavelength, the light was only sensitive to the overall density of the sheet, which the researchers could control by drilling more holes. The more holes, the less silicon there is to bend the light, and therefore the less the light is bent as it passes through. By introducing changes in density and therefore in how much light is bent in a particular region of the sheet, the scientists succeeded in coaxing the light to flow in a curved line around an object.

And earlier this year, the concept was extended to a three-dimensional cloak by a team from the Karlsruhe Institute of Technology in Germany, who constructed a polymer crystal made up of tiny rods. By varying the thickness of the rods, it was possible to tailor the extent to which different parts of the material bend light as it passes through, allowing an object to appear invisible from a viewing range of around 60 degrees. This is a big improvement on the holes in the silicon sheet, which only hide an object from light travelling in the same direction as the drilling with the result that the cloak will only be effective if you’re hiding from someone standing very still and directly in front of you!

The science of invisibility continues to develop, as just this week, it has been shown that it is theoretically possible to open up a time gap in a light signal in which nefarious activities can be hidden. Still, there’s a long way to go for invisibility cloaks yet. The biggest challenges are scaling the cloaks up (both the silicon and polymer cloaks can only hide micron-scale objects) and getting the wavelength down (to pass from the near-infrared spectrum to the higher-frequency radiation visible to humans). However, researchers are increasingly optimistic about overcoming these challenges so invisibility may well be worth keeping an eye on.