To visualize how the radiation bends around the device, think about the way a river flows around a boulder. Because the water quickly wraps around the boulder, the rock is undetectable downstream. Similarly, metamaterials can continuously alter and bend the path of microwaves so that they flow around a cylinder, for example, essentially making everything inside the cylinder invisible to microwaves [see illustration below].
In general, the internal structures implanted inside such a metamaterial must be smaller than the wavelength of the radiation to be redirected. For example, for a metamaterial to bend the path of microwaves with a wavelength of three centimeters, it must have tiny implants embedded in it that are smaller than three centimeters. But to make an object invisible to green light, with a wavelength of 500 nanometers (nm), the metamaterial must have structures embedded within it that are only about 50 nm long. (One nanometer is a billionth of a meter in length. Approximately five atoms fit within a single nanometer.) Building such atomic-scale structures requires advanced nanotechnology at the very limits of modern engineering. Pushing those limits is perhaps the key problem engineers face in their attempts to create a true invisibility cloak.
The rac is on—just as it is in the computer-chip industry—to use nanotechnology to achieve better performance at smaller scales. Ever since the announcement that metamaterials have been fabricated in the laboratory, there has been a stampede of activity in this area, with new insights and startling breakthroughs coming every few months. And the goal is to create metamaterials that can bend visible light, not just microwaves.
Advances made in the semiconductor industry have spurred the creation of new metamaterials. A technique called “optical lithography” enables engineers to place hundreds of millions of tiny transistors onto a silicon wafer no bigger than your thumbnail. Scientists use ultraviolet radiation to “etch” tinier and tinier components onto a silicon chip. At present, the smallest components that one can create with this etching process are about 30 nm (or about 150 atoms) across. With the etching technology, a group of scientists created the first metamaterial that operates in the visible range of light: scientists in Germany and at the U.S. Department of Energy (DOE) announced in late 2006 that, for the first time in history, they had fabricated a metamaterial that worked for red light. How did they achieve the “impossible”? Physicist Costas M. Soukoulis of the DOE’s Ames Laboratory in Iowa joined forces with Gunnar Dolling and Martin Wegener of the University of Karlsruhe and Stefan Linden of the Karlsruhe Research Institute in Germany. They started with a glass sheet, and then deposited a thin coating of silver, followed by magnesium fluoride and another layer of silver, forming a fluoride “sandwich” that was only 100 nm thick. Next, using standard etching techniques, they made a large array of microscopic square holes, 100 nm wide, in the sandwich to create a grid pattern resembling a fishnet. Then they passed a red light beam with a wavelength of 780 nm through the material and measured its refractive index, which was -0.6. (Previously, the smallest wavelength of radiation bent by a metamaterial was 1,400 nm, which put it outside the visible light spectrum, in the range of infrared.)
Physicists foresee many applications of this invisibility technology. Metamaterials “may one day lead to the development of a type of flat superlens that operates in the visible spectrum,” says Soukoulis. “Such a lens would offer superior resolution over conventional technology, capturing details much smaller than one wavelength of light.” Breakthrough applications of such a “superlens” would include the photographing of microscopic objects, such as the inside of a living human cell, with unparalleled clarity; it could make possible the early diagnosis of diseases in a developing baby inside the womb. Ideally, one would be able to take photographs of a DNA molecule without having to use clumsy X-ray crystallography.
So far, Soukoulis’s group has only been able to manipulate red light. The next step would be to create a metamaterial that would bend red light entirely around an object, rendering it invisible to that light.