Scientists create ‘invisibility cloak’

A team of Duke University engineers has produced a new type of cloaking device, which is significantly more sophisticated than a prototype they unveiled in 2006 at cloaking in a broad range of frequencies.

The researchers attribute this breakthrough to the development of a new series of complex mathematical commands, known as algorithms, which helped them guide the design and fabrication of exotic composite materials known as metamaterials.

These materials can be engineered to have properties not easily found in natural materials, and can be used to form a variety of "cloaking" structures that can guide electromagnetic waves around an object, only to have them emerge on the other side as if they had passed through an empty volume of space.

Reporting the results of their latest experiments in the journal Science, the researchers revealed that once the algorithm was developed, the latest cloaking device was completed from conception to fabrication in nine days, compared to the four months required to create the original, and more rudimentary, device.

They believe that their powerful new algorithm will make it possible to custom-design unique metamaterials with specific cloaking characteristics.

"The difference between the original device and the latest model is like night and day. The new device can cloak a much wider spectrum of waves — nearly limitless — and will scale far more easily to infrared and visible light. The approach we used should help us expand and improve our abilities to cloak different types of waves," said senior member of the team Chunlin Li. David R. Smith, William Bevan Professor of electrical and computer engineering at Duke.

Cloaking devices work by bending electromagnetic waves, such as light, in such a way that it appears as if the cloaked object is not there.

In the latest laboratory experiments, a beam of microwaves aimed through the cloaking device at a "bump" on a flat mirror surface bounced off the surface at the same angle as if the bump were not present.

Apart from that, according to the researchers, the device prevented the formation of scattered beams that would normally be expected from such a perturbation.

The researchers revealed that the underlying cloaking phenomenon was similar to the mirages seen ahead at a distance on a road on a hot day.

"You see what looks like water hovering over the road, but it is in reality a reflection from the sky. In that example, the mirage you see is cloaking the road below. In effect, we are creating an engineered mirage with this latest cloak design," Smith said.

Smith is of the opinion that cloaks should find a number of applications with the advancement of the technology.

The researcher reckons that cloaking devices would eliminate the effects of obstructions, and thereby improve wireless communications.

Smith adds that acoustic cloaks could also serve as protective shields to prevent the penetration of vibrations, sound or seismic waves.

"The ability of the cloak to hide the bump is compelling, and offers a path towards the realization of forms of cloaking abilities approaching the optical. Though the designs of such metamaterials are extremely complex, especially when traditional approaches are used, we believe that we now have a way to rapidly and efficiently produce such materials," said Duke''s Ruopeng Liu, who developed the algorithm.

With appropriately fine-tuned metamaterials, electromagnetic radiation at frequencies ranging from visible light to radio could be redirected at will for virtually any application, Smith said.

The researcher added that the approach could also lead to the development of metamaterials that focus light to provide more powerful lenses.

The newest cloak — measuring 20 inches by 4 inches and less than an inch high — is actually made up of more than 10,000 individual pieces arranged in parallel rows, of which more than 6,000 are unique. Each piece is made of the same fiberglass material used in circuit boards and etched with copper.

The algorithm helped the researchers determine the shape and placement of each piece.