"By incorporating complex material properties, our cloak allows a concealed volume, plus the cloak, to appear to have properties similar to free space when viewed externally," said David R Smith of Duke University. "The cloak reduces both an object's reflection and its shadow, either of which would enable its detection."

The cloak consisted of 10 concentric two-dimensional rings joined by six radial spokes. The structure was made of Durion composite (glass fibre-reinforced PTFE) patterned with a 17 µm thick layer of copper. The copper was patterned into split-ring resonators of various dimensions.

"Unlike other metamaterials, the cloak requires a gradual change in its properties as a function of position," said Smith. "Achieving that gradient in material properties was a fairly significant design effort."

The team used the cloak to hide a 25 mm radius copper cylinder from a narrow microwave beam. Field mapping revealed that the wave front separated to pass around the cloak hole and reformed on the opposite side.

"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," said David Schurig of Duke University. "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."

Currently the cloak only works for a narrow range of microwave frequencies and creates imperfect invisibility due to the presence of slight reflection and shadowing.

"One would like to say that we will be making all sorts of things invisible, but unfortunately it isn't so straightforward," explained Smith. "The entire effect that we are looking at is inherently narrow-band, meaning we can ideally only cloak a single frequency and then maybe some small bandwidth around that. When we think of making something invisible to our eye, we would have to cloak the object over a relatively large spectral range – 400–700 nm."

Smith says that even if the team could make the structure at those wavelengths, it would not be able to cloak the entire range. "At best, we could cloak one colour and this could be interesting, but we run into a second problem: the metals that work so well for us at microwave frequencies become very absorptive at visible wavelengths, so our scaled-down cloak would actually appear entirely opaque. This is a problem we don't know how to solve yet, but it is not insurmountable in principle."

The team's existing cloak technology could be compatible with the frequencies of many communications bands, which also have the advantage of being quite narrow. "At the moment, we do not have any particular application in mind, but there may be cases where, rather than hiding an object from detection, you might want to remove it as an impediment," said Smith. "For example, if there is something that is interfering with communications between two points, one could coat the structure with this cloak and remove it as an obstacle."

Now the team plans to create a more advanced version of their two-dimensional cloak, which will further reduce reflection and shadowing, and to develop a three-dimensional cloak.

The researchers reported their work in Sciencexpress.