A team led by scientists at Duke University's Pratt School of Engineering has demonstrated the first working "invisibility cloak." 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. By incorporating complex material properties, the cloak allows a concealed volume, plus the cloak, to appear to have properties similar to free space when viewed externally. The cloak reduces both an object's reflection and its shadow, either of which would enable its detection.
The researchers manufactured 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. The cloak represents one of the most elaborate metamaterial structures yet designed and produced, and also represents the most comprehensive approach to invisibility yet realized, with the potential to hide objects of any size or material property. The team produced the cloak according to electromagnetic specifications determined by a new design theory proposed by Sir John Pendry of Imperial College London, in collaboration with the Duke scientists. The principles behind the cloaking design, though mathematically rigorous, can be applied in a relatively straightforward way using metamaterials.
One first imagines a distortion in space similar to what would occur when pushing a pointed object through a piece of cloth, distorting, but not breaking, any threads. In such a space, light or other electromagnetic waves would be confined to the warped 'threads' and therefore could not interact with, or 'see,' objects placed inside the resulting hole. The researchers used a mathematical description of that concept to develop a blueprint for a cloak that mimics the properties of the imagined, warped space. You cannot easily warp space, but you can achieve the same effect on electromagnetic fields using materials with the right response. The required materials are quite complex, but can be implemented using metamaterial technology. 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 fiberglass composite that are traditionally used in computer circuit boards.
To simplify design and fabrication in the current study, the team set out to develop a small cloak, less than five inches across, that would provide invisibility in two dimensions, rather than three. In essence, the cloak includes strips of metamaterial fashioned into concentric two-dimensional rings, a design that allows its use with a narrow beam of microwave radiation. The precise variations in the shape of copper elements patterned onto their surfaces determine their electromagnetic properties. The cloak design is unique among metamaterials in its circular geometry and internal structural variation. All other metamaterials have been based on a cubic, or gridlike, design, and most of them have electromagnetic properties that are uniform throughout. Unlike other metamaterials, the cloak requires a gradual change in its properties as a function of position. Rather than its material properties being the same everywhere, the cloak's material properties vary from point to point and vary in a very specific way. Achieving that gradient in material properties was a fairly significant design effort.
To assess the cloak's performance, the researchers aimed a microwave beam at a cloak situated between two metal plates inside a test chamber, and used a specialized 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 center of the cloak. The waves' movement is similar to river water flowing around a smooth rock. Moreover, the observed physical behavior of the cloak proved to be in "remarkable agreement" with that expected based on a simulated cloak. Although the new cloak demonstrates the feasibility of the researchers' design, the findings nevertheless represent a "baby 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. To make an object literally vanish before a person's eyes, a cloak would have to simultaneously interact with all of the wavelengths, or colors, that make up light, he said. That technology would require much more intricate and tiny metamaterial structures, which scientists have yet to devise. Cloaks that render objects essentially invisible to microwaves could have a variety of wireless communications or radar applications.
Earlier scientific approaches to achieving "invisibility" often relied on limiting the reflection of electromagnetic waves. In other schemes, scientists attempted to create cloaks with electromagnetic properties that, in effect, cancel those of the object meant to be hidden. In the latter case, a given cloak would be suitable for hiding only objects with very specific properties. Huanyang Chen of Shanghai Jiao Tong University and his colleagues have proposed a theoretical "anti-cloak" that would partially cancel the effect of the invisibility cloak. Meta materials are effectively invisible because of the way they interact with light. All materials scatter, bounce, absorb, reflect and otherwise alter light rays that strike them. Transformation media cloaks are special materials that can bend light so much that it actually passes around the object completely. Invisibility as it has been achieved so far in the laboratory is very limited. It works, but only for a narrow band of light wavelengths. An even greater problem for anyone who has aspirations to be concealed in public one day is that invisibility achieved through transformation media is a two-way street. With no light penetrating a perfect invisibility cloak, there would be no way for an invisible person to see outside. In other words, invisible people would also be blind. The "anti-cloak" would be a material with optical properties perfectly matched to those of an invisibility cloak. (In technical jargon, an anti-cloak would be anisotropic negative refractive index material that is impedance matched to the positive refractive index of the invisibility cloak). While an invisibility cloak would bend light around an object, any region that came into contact with the anti-cloak would guide some light back so that it became visible. This would allow an invisible observer to see the outside by pressing a layer of anti-cloak material in contact with an invisibility cloak.