A review on cloaking theory

Scientists and novelists have been intrigued for centuries by the possibility of hiding an object so completely that neither trace of the object nor of its cloak is to be found. Recent theoretical developments show that cloaking is, in principle, possible for electromagnetic waves and to a limited extent for other types of wave, such as acoustic waves. An energetic program of experimental research has shown some of the schemes to be realizable in practice.

We have a touching faith in the ability of our eyes to tell the truth. No other sense has such confidence invested in it, so when our eyes deceive us the result is bewilderment, giving rise to appeals to magic or even the supernatural. This explains the enormous interest aroused by recent work on invisibility and the cloaking of objects from electromagnetic radiation. In this article we review the theories and experiments behind the hype and suggest what devices might realistically be expected in the near future and what is likely to prove impossible.

Hard wired into our brains is the expectation that light travels in straight lines. Mostly this is true, but there are well-known exceptions, such as mirages, which occur when a hot surface heats the air above, reducing its density and hence creating a refractive index gradient immediately above the surface (Fig. 1, top). Such a gradient bends the trajectories of light rays so that an observer misinterprets where the light is coming from. Typically, light from the sky is refracted by the gradient, giving the appearance of water shimmering in the distance—hence a cruel illusion seen by a thirsty traveler in the desert or, more prosaically, the appearance of a wet road on a hot day.

It is the ability of refractive index gradients to bend light that the invisibility engineer exploits. Light is steered around the hidden object by a cloaking device, and then returned to the same straight line trajectory, rather as a skier would make a chicane around a tree (Fig. 1, bottom). The observer’s brain is unaware of the possibility of chicanes and sees only that which is behind the cloak and nothing of the cloak itself or of its contents. The real challenge of cloaking lies in deriving a theoretical prescription for the optical properties of the cloak and, even more challenging, realizing these properties in a material. Transformation optics provides the theoretical background and metamaterials provide the means of achieving the prescribed parameters.