November 11, 2012

Full-parameter unidirectional metamaterial cloak for microwaves

Nature Materials - Invisibility is a notion that has long captivated the popular imagination. However, in 2006, invisibility became a practical matter for the scientific community as well, with the suggestion that artificially structured metamaterials could enable a new electromagnetic design paradigm, now termed transformation optics1. Since the advent of transformation optics and subsequent initial demonstration of the microwave cloak, the field has grown rapidly. However, the complexity of the transformation optics material prescription has continually forced researchers to make simplifying approximations to achieve even a subset of the desired functionality. These approximations place profound limitations on the performance of transformation optics devices in general11, and cloaks especially. Here, we design and experimentally characterize a two-dimensional, unidirectional cloak that makes no approximations to the underlying transformation optics formulation, yet is capable of reducing the scattering of an object ten wavelengths in size. We demonstrate that this approximation-free design regains the performance characteristics promised by transformation optics.

"One issue, which we were fully aware of, was loss of the waves due to reflections at the boundaries of the device," Landy said. He explained that it was much like reflections seen on clear glass. The viewer can see through the glass just fine, but at the same time the viewer is aware the glass is present due to light reflected from the surface of the glass. "Since the goal was to demonstrate the basic principles of cloaking, we didn't worry about these reflections." Landy has now reduced the occurrence of reflections by using a different fabrication strategy. The original cloak consisted of parallel and intersecting strips of fiberglass etched with copper. Landy's cloak used a similar row-by-row design, but added copper strips to create a more complicated—and better performing—material. The strips of the device, which is about two-feet square, form a diamond-shape, with the center left empty. When any type of wave, like light, strikes a surface, it can be either reflected or absorbed, or a combination of both. In the case of earlier cloaking experiments, a small percentage of the energy in the waves was absorbed, but not enough to affect the overall functioning of the cloak. The cloak was naturally divided into four quadrants. Landy explained the "reflections" noted in earlier cloaks tended to occur along the edges and corners of the spaces within and around the meta-material. "Each quadrant of the cloak tended to have voids, or blind spots, at their intersections and corners with each other," Landy said. "After many calculations, we thought we could correct this situation by shifting each strip so that it met its mirror image at each interface. "We built the cloak, and it worked," he said. "It split light into two waves which traveled around an object in the center and re-emerged as the single wave with minimal loss due to reflections." Landy said this approach could have more applications than just cloaks. For example, meta-materials can "smooth out" twists and turns in fiber optics, in essence making them seem straighter. This is important, Landy said, because each bend attenuates the wave within it. The researchers are now working to apply the principles learned in the latest experiments to three dimensions, a much greater challenge than in a two-dimensional device.

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