Making the visible invisible, and vice versa
More than a century ago, H.G. Wells told the fictional tale of Griffin, a gifted medical student who managed to make himself disappear. Griffin became the Invisible Man by tinkering with his body’s refractive index, the measure of how light is deflected off an object. Last fall, Berkeley researchers announced the development of new materials that have revolutionary refractive properties, bringing the prospect of invisibility from the pages of science fiction to the pages of science journals. Someday, these pioneering efforts may even lead to a real-life version of Harry Potter’s invisibility cloak.
Mechanical engineering professor Xiang Zhang, M.E. ‘96, and his colleagues are developing metamaterials—composites that can bend electromagnetic waves, including light, in bizarre ways. The magic lies not in the ingredients used to make these materials, but in their nanoscale structures. (A nanometer is a billionth of a meter.) The metamaterials are capable of doing something many scientists thought impossible: bending light backwards.
“We’ve shown that it’s possible to design new composite materials with extraordinary properties not found in nature,” says Zhang, director of Berkeley’s Nanoscale Science and Engineering Center.
Not that nature has any problem bending light. Plunge a stick into a lake, and the part of the stick that’s underwater will appear to bend upward toward the surface. That’s positive refraction. But if the lake were to have a negative refractive index like Zhang’s metamaterials, the stick would look as if it were jutting out of the water, and fish would appear to fly over the surface. Fortunately for our sanity, negative refractive indexes are found only in the laboratory.
To produce negative refraction, the Berkeley researchers fabricated two kinds of bulk metamaterials. The first is composed of tiny nanowires, each one 1,600 times thinner than a human hair, embedded inside porous aluminum oxide. As near-infrared light passes through the nanowires, the waves bounce backwards. The other kind of bulk metamaterial consists of layered nonmagnetic silver and magnesium fluoride perforated in a fishnet pattern. The alternating layers form a medium in which the electromagnetic waves of light travel backwards. Professor Zhang’s Xlab was the first to demonstrate this phenomenon with visible light.
“The ability to bend light in any direction could certainly lead to an invisibility cloak,” Zhang says. Perhaps not the actual cloak, he allows, but “the fundamental physics are there.”
Right now, Zhang’s group is conducting proof-of-concept experiments they hope will demonstrate that the metamaterials could have real-world applications. No, Zhang isn’t planning to follow in Griffin’s footprints. “We’re not trying to cloak a person, a house, or a tank,” he says. Rather, he and his colleagues are trying to cloak the equivalent of a kink in a carpet, albeit smaller than the naked eye can see. Imagine a microscale rug that’s been unrolled but isn’t lying perfectly flat. If the experiment proves successful, metamaterials will cause the lump, roughly 1/20 the thickness of a human hair, to vanish under the microscope. If they can do it at the microscopic level, Zhang says, there’s no reason it couldn’t be scaled up.
Meanwhile, Zhang’s group has also set its sights on more immediate applications for the metamaterials, starting with optical lithography, which is the technology used to manufacture computer chips. Traditional optical lithography is hampered by a physical constraint called the diffraction limit (the maximum resolution a lens can achieve). Metamaterials’ light-bending properties topple the diffraction limit, enabling a lens with theoretically perfect focus: a superlens. By focusing optical waves so minutely, semiconductor manufacturers could pattern much tinier circuits into silicon chips. The result would be much denser microprocessors that are far more powerful than today’s computers.
Look at the superlens another way, though, and it becomes an anticloaking device, revealing worlds now hidden to us. With a superlens microscope, scientists could see the most minuscule machinations of living cells. Indeed, it could provide biologists with a window directly inside a cell, to observe drug interactions at the protein level.
“The fundamental science here is vast,” Zhang says. “But as an engineer, I’m driven by the extraordinary properties of these metamaterials and the potential of the technology. Our research is just the tip of the iceberg.”