A mainstay of science fiction and prized possession of the young wizard Harry Potter, the invisibility cloak is also of great interest to the defence and intelligence communities. As a result, a great deal of research (both secret and public) has gone into the development of materials that can hide objects from prying eyes – or at least radar systems. Now researchers in the US and the UK have claimed the first public success (Scienceexpress 1133628).
David Smith and colleagues at Duke University in North Carolina have used specially-structured materials called metamaterials to create a device that can make an object almost invisible to the microwave radiation used in some radar systems. Based on a design by the physicist John Pendry of Imperial College, the cloak bends microwave radiation around the object, like water flowing around a smooth stone. This makes both the cloak and object invisible to an observer because the radiation does not appear to be scattered or absorbed by cloak or object.
The cylindrical cloak has a radius of about 6 cm and surrounded a copper cylinder (the object). The arrangement was exposed to microwave pulses at about 10 GHz and a moveable antenna was used to study how the cloak interacted with the radiation. Although the cloak did absorb and scatter some of the microwave radiation, the pulses did bend around the object and reform on the other side.
The cloak is a cylindrical arrangement of split-ring resonators (SRRs), which are made out of thin strips of copper and resemble a square-shaped ring that is partially split into two rectangles. The SSRs were arranged in ten concentric rings, each three resonators tall.
By varying the shape, size and arrangement of the resonators, the researchers were able to engineer the electrical permittivity and magnetic permeability at any point within the cloak. The theory of transformation optics was used to determine which permittivity and permeability values would steer the microwave radiation smoothly around the object. Transformation optics is a new and expanding field of physics that has been made possible by the development of metamaterials.
The cylindrical nature of the cloak means that it can only shield in two dimensions and the current design only works within a relatively narrow microwave frequency band. The researchers are currently working on a spherical device that could cloak in all directions. However, an invisibility cloak that works with visible light will remain science fiction for the time being, because it would require much smaller and more intricate structures that have yet to be devised, say the researchers.
