Pictured is the new cloak with bump, left, and the prototype, right. (Image: Duke University)
Invisibility, cloaking, superlenses, hyperlenses, super-microscopes, advanced lithography are all closer to being practically achieved.
New algorithms were developed and 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. This powerful new algorithm will make it possible to custom-design unique metamaterials with specific cloaking characteristics, the researchers said.
"The difference between the original device and the latest model is like night and day," Smith said. "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."
MIT Technology Review describes the new breakthrough
The cloak that the researchers built works with wavelengths of light ranging from about 1 to 18 gigahertz--a swath as broad as the visible spectrum. No one has yet made a cloaking device that works in the visible spectrum, and those metamaterials that have been fabricated tend to work only with narrow bands of light. But a cloak that made an object invisible to light of only one color would not be of much use. Similarly, a cloaking device can't afford to be lossy: if it lets just a little bit of light reflect off the object it's supposed to cloak, it's no longer effective. The cloak that Smith built is very low loss, successfully rerouting almost all the light that hits it.
The broadband cloak is a rectangular structure measuring about 50 by 10 centimeters, with a height of about 1 centimeter. It's made up of roughly 600 I-shaped copper structures. Making each structure is a simple matter, says Smith. "They're copper patterns on a circuit board, cut up and arranged. It's a well-known, inexpensive technology." The hard part is determining the dimensions of each of these 600 structures and how to arrange them.
The cloak itself, described this week in Science, is indeed impressive, says Fang, who's working on metamaterials for super-resolution biological imaging. But what's more exciting is that the new approach to design will accelerate the development of other metamaterials. Smith says that he and his group have already moved beyond the cloak reported in Science, but because their latest work is unpublished, he can't specify what they've made. "Now [that] this is becoming a more feasible technology," he says, "we will start to see a lot more of it."
Other applications of metamaterials, says Smith, include optical devices that take light energy and concentrate it, instead of turning it away--conceptually, the opposite of a cloak. "You could improve solar cells by making structures to increase the field strength of the light," he says. The new work suggests that this could be done over the whole spectrum of wavelengths found in sunlight. Similarly, broadband "hyperlenses" that gather up light missed by normal lenses could revolutionize biological imaging. Fang and others have developed narrowband hyperlenses with resolutions of only a few nanometers, which make the molecular workings of cells visible. A broadband hyperlens could work with all colors of visible and infrared light.
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. Additionally, the device prevented the formation of scattered beams that would normally 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 explained. "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 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," Liu said. "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 could also lead to the development of metamaterials that focus light to provide more powerful lenses.
The newest cloak, which measures 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 those pieces, 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 determined the shape and placement of each piece. Without the algorithm, properly designing and aligning the pieces would have been extremely difficult, Smith said.