Xiang Zhang runs
the cutting-edge metamaterials
lab at the University
of California at Berkeley.
Peg Skorpinski
The same problem crops up everywhere. For instance, the advancement of the microcomputer industry depends on circuits with smaller and smaller features, but we must still use some form of light to print those features. Past a certain point—one we are very close to reaching—features become too little to be printed. The task becomes like using a crayon to write very fine letters. The farther we walk down this tunnel, the narrower it gets, and these days it is getting quite cramped indeed.
Pendry envisioned a way out of the tunnel, with metamaterials to guide the way. Tiny objects are difficult to observe because they have almost no reflection to focus on. But Pendry realized that when light hits a small object, the impacting radiation triggers a subtle effect that manifests itself as a pattern of local waves. The waves vanish without a trace almost immediately after being generated. Pendry thought that if metamaterials were built and positioned just right, they could pick up, preserve, and process these evanescent waves, converting them into a form that could be resolved into useful images. Normal lenses cannot produce images from such waves because of the limitations of how they bend light. But metamaterials can transcend those limits, providing a tight focus that no piece of glass (or any other ordinary material) ever could.
Pendry still remembers the day he realized that this new kind of lens would allow people to see invisibly small objects. “I was astonished and, frankly, frightened to publish the conclusion,” he says, “because I knew that everyone’s first reaction would be that it could not be true. I can tell you that I checked the result very many times!”
In 2004 researchers at the University of Toronto created the first superlens to tap into the minute details hidden in those evanescent waves, although the lens could focus only microwave radiation. The following year Zhang’s team crafted an ultrathin layer of silver into the first optical superlens. Its resolution was several times better than that of the best optical microscopes. And in 2007 Zhang did even better by developing a lens that amplified the signal of the evanescent waves. The lens clearly resolved two nanowires separated by 150 nanometers, a gap narrower than a single wave of visible light. Pendry has no doubt that the “perfect lenses” he conceived will get steadily more powerful. “Fundamental physics sets no limits,” he says.
Pendry’s work on metamaterials is notoriously complicated and serious, so it is ironic that the most famous implication of his research—the invisibility cloak—began as a joke. Heeding the suggestion of a colleague, he decided to have some fun with a lecture he was delivering at a 2004 DARPA meeting. He knew metamaterials could theoretically hide objects from sight, so he made a Harry Potter reference—but not the one you might expect. He brought up Platform 9¾, the invisible departure point for Harry’s trip to Hogwarts, drawing some laughs from the crowd. “And then the DARPA woman started taking things seriously,” Pendry says. “She offered me half a million dollars to work on it.”
Soon afterward, Smith approached Pendry to tell him that he would like to build a cloak for real. “I told Smith he must be crazy, and then he did it,” Pendry says, chuckling.
In 2006, ensconced in his new lab at Duke University, Smith developed the first functional invisibility cloak, though not for visible light. The cloak steered radiation around a copper cylinder to hide the object from microwave detection.
From there, Zhang picked up the ball. By then he was set up at Berkeley, running a 40-person research group. While the projects Zhang oversees are varied, at the group’s core is a metamaterial fabrication shop, the most ambitious of its kind in the world. Foreshadowing the jolt that metamaterials is likely to give the general field of 3-D nanofabrication, the facility builds really small but geometrically complex devices.
That skill is necessary to get us to full-blown nanotechnology and to the long list of amazing devices (like universal fabricators, countertop food factories, intracellular longevity boosters, even telepathy implants) that it might make possible. However, almost all of engineers’ present expertise in building small objects involves two-dimensional structures, such as microprocessors and computer memory chips—in part because digital electronics generate lots of heat, and it is easier to cool something that is two-dimensional. Engineers have been speculating about 3-D chip architectures for years, but the change in skills and tools needed to build such devices is so radical that the feat has never found investment sufficient to do the job.
Until now.
The metamaterial known as
fishnet, produced in Zhang's lab, could
guide the design of an invisibility cloak in
years to come.
Jason Valentine UC Berkeley
With his metamaterials workshop, Zhang is the harbinger of change. Since so many metamaterials applications are inherently 3-D, the trickle-down from his work will be vast. For example, building a useful invisibility cloak—the kind that could hide a person or a military tank—requires crafting many little devices that pick up a ray of light on the far side of an object, away from the observer, and then relay that ray, row by row, around the object. When the ray arrives at the side facing the observer, it is re-emitted in the direction it would have taken had the object not been there at all. Very weird—and a decade ago it would have been considered a total violation of the laws of refraction—but seemingly doable. For the cloak to be useful, though, it would have to bend light from all directions. To do that, a 3-D array of light-bending devices is a must.
It was in the summer of 2008 that Zhang’s group took the first small step toward creating a real-world invisibility cloak by fabricating the first optical metamaterial to work in three dimensions. Made of 21 alternating sheets of silver and a glasslike substance, the material, dubbed fishnet, contains rectangular holes that resemble waffles or sieves. As light travels through the fishnet, the alternating layers act as circuits that bend light in unusual ways. A separate group in Zhang’s lab accomplished a similar feat using silver nanowires embedded in a solid base. In principle, a more durable version of these materials should be able to guide light around an object, creating the desired cloak.
This past January, Smith built a structure that comes even closer to the goal of building an invisibility cloak large enough to hide a person. The device is almost two feet long and four inches wide, with a small bump on one of the thin edges. If you place a small object beneath the bump and then shine microwave radiation on the cloak, the microwaves that bounce back will look like reflections from a blank mirror—as if the bump (and anything hidden beneath it) were not there.
The breakthrough in this work: Smith’s metamaterial device is the first to handle a very wide range of wavelengths, a necessity for Harry Potter–level invisibility. His design shows it is possible to cloak all the colors of an object at once, from red to blue and everything in between. The device is still a far cry from a wearable cloak—it works only on a flat surface, and it responds to microwaves, not light—but it is a big step in that direction. Smith says the design should easily scale down to the visible spectrum, meaning that soon we may see a “bump” of matter seemingly disappear in front of our eyes. Smith is betting that someone will build such a device by the end of this summer.
