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Metamaterials Hold Promise For Invisibility Cloaks

Researchers used 3-D metamaterials, or composites with the ability to bend electromagnetic waves, to negatively refract light.

Researchers at the University of California, Berkeley, say they have created materials that bring them closer to creating invisibility cloaks.

They will publish their work in two journals, Nature and Science, this week. The researchers used 3-D metamaterials, or composites with the ability to bend electromagnetic waves, to negatively refract light.

"What we have done is take two very different approaches to the challenge of creating bulk metamaterials that can exhibit negative refraction in optical frequencies," Xiang Zhang, professor at Berkeley's Nanoscale Science and Engineering Center, said in a statement. "Both bring us a major step closer to the development of practical applications for metamaterials."

Zhang heads the research teams that developed the two new metamaterials. He also is a faculty scientist in the Material Sciences Division at the Lawrence Berkeley National Laboratory.

The Berkeley researchers stacked layers of silver and nonconducting magnesium fluoride and cut nanoscale-sized fishnet patterns into them for the metamaterials. They measured a negative index of refraction at wavelengths as short as 1,500 nanometers, or the near-infrared light range, according to a university announcement.

The researchers also demonstrated a second method of negatively refracting light. They used silver nanowires grown inside porous aluminum oxide to create a structure about one-tenth the thickness of a sheet of paper. The metamaterial refracted red light wavelengths as short as 660 nanometers, according to the researchers.

"The geometry of the vertical nanowires, which were equidistant and parallel to each other, were designed to only respond to the electrical field in light waves," explained Jie Yao, a student in UC Berkeley's Graduate Program in Applied Science and Technology and co-lead author of the study in Science. "The magnetic field, which oscillates at a perpendicular angle to the electrical field in a light wave, is essentially blind to the upright nanowires, a feature which significantly reduces energy loss."

"What makes both these materials stand out is that they are able to function in a broad spectrum of optical wavelengths with lower energy loss," Zhang said. "We've also opened up a new approach to developing metamaterials by moving away from previous designs that were based upon the physics of resonance. Previous metamaterials in the optical range would need to vibrate at certain frequencies to achieve negative refraction, leading to strong energy absorption. Resonance is not a factor in both the nanowire and fishnet metamaterials."

Scientists hope the research can lead to improved antenna performance, reversing the Doppler effect, higher-resolution optical imaging, nanocircuits for high-powered computers, and, eventually, cloaking devices that could render objects invisible to humans.

However, they cautioned that they are far from being able to create invisibility cloaks.

The National Science Foundation, the U.S. Army Research Office, and the U.S. Air Force Office of Scientific Research helped fund the research.

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