A team led by scientists atDuke University's Pratt School of Engineering
has demonstrated the first working ‘invisibility cloak’ that effectively render objects invisible to microwaves.
The cloak deflects microwave beams so they flow around a ‘hidden’ object inside with little distortion, making it appear almost as if nothing were there at all. It could have a variety of wireless communications or radar applications, according to the researchers.
They manufactured a two-dimensional version of the cloak using ‘metamaterials’ precisely arranged in a series of concentric circles that confer specific electromagnetic properties. Metamaterials are artificial composites that can be made to interact with electromagnetic waves in ways that natural materials cannot reproduce
While the properties of natural materials are determined by their chemistry, the properties of metamaterials depend instead on their physical structure. In the case of the new cloak, that structure consists of copper rings and wires patterned onto sheets of fibreglass composite that are traditionally used in computer circuit boards.
The cloak design is unique among metamaterials in its circular geometry and internal structural variation, the researchers said. All other metamaterials have been based on a cubic, or grid-like, design, and most of them have electromagnetic properties that are uniform throughout.
The team produced the cloak according to electromagnetic specifications determined by a design theory proposed by Sir John Pendry of Imperial College London, in collaboration with the Duke scientists.
The researchers said the cloak acts like a hole has been poked in space. All light or other electromagnetic waves are swept around the area, guided by the metamaterial to emerge on the other side as if they had passed through an empty volume of space.
To assess its performance, the researchers aimed a microwave beam at the cloak situated between two metal plates inside a test chamber, and used a specialised detecting apparatus to measure the electromagnetic fields that developed both inside and outside the cloak. By examining an animated representation of the data, they found that the wave fronts of the beam separate and flow around the centre of the cloak.
Although the new cloak demonstrates the feasibility of the researchers' design, the findings nevertheless represent a step on the road to actual applications for invisibility. The researchers said they plan to work toward developing a three-dimensional cloak and further perfecting the cloaking effect.
Although the same principles applied to the new microwave cloak might ultimately lead to the production of cloaks that confer invisibility within the visible frequency range, that eventuality remains uncertain, the researchers said.