New Cloak vs. Cloak Classic

A new type of device that can render an object invisible in visual light has been revealed by the same research team that constructed the first prototype in 2006. The new device is significantly more sophisticated at cloaking in a broad range of frequencies.

David R. Smith with the new cloak device. Images courtesy of Duke University Photography.

“The difference between the original device and the latest model is like night and day,” said David R. Smith, William Bevan professor of electrical and computer engineering at Duke University and senior member of the research team. “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.”

The latest advance was made possible by the development of a new series of complex algorithms to 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. The structures 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.

Once the algorithm was developed, a cloaking device was completed from conception to fabrication in nine days, compared with the four months required to create the original, and more rudimentary, device. The powerful new algorithm will make it possible to design unique metamaterials with specific cloaking characteristics, the researchers say.

Cloaking devices bend electromagnetic waves, such as light, in such a way that it appears as if the cloaked object is not there. In the latest lab 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 it would have if the bump had not been present. Additionally, the device prevented the formation of scattered beams that normally would be expected from such a perturbation.

The underlying cloaking phenomenon is 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," Smith said. “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 believes that cloaks should find numerous applications as the technology is perfected. By eliminating the effects of obstructions, cloaking devices could improve wireless communications, or acoustic cloaks could serve as protective shields, preventing the penetration of vibrations, sound or seismic waves.

The new cloak with bump (left) and the prototype (right).

“The ability of the cloak to hide the bump is compelling and offers a path toward the realization of forms of cloaking abilities approaching the optical,” said Duke’s Ruopeng Liu, who developed the algorithm. “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.”

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

The newest cloak, which measures 20 × 4 in. and less than 1 in. high, is actually made up of more than 10,000 individual pieces arranged in parallel rows. Of those pieces, more than 6000 are unique. Each piece is made of the same fiberglass material used in circuit boards and etched with copper.

The algorithm determined the shape and placement of each piece. Without the algorithm, properly designing and aligning the pieces would have been extremely difficult, Smith said.

The results of the latest experiments will be published Jan. 16 in the journal Science; first authors of the paper were Liu and Chunlin Li.

The research was supported by Raytheon Missile Systems, the Air Force Office of Scientific Research, InnovateHan Technology, the National Science Foundation of China, the National Basic Research Program of China and the National Science Foundation of Jiangsu Province, China.

Others members of the research team were Jack Mock of Duke, and Jessie Y. Chin and Tie Jun Cui of Southeast University in Nanjing, China.

For more information, visit: http://www.duke.edu/