After that occurs, there will still be one big limitation: Anyone inside the invisibility cloak would not be able to see out, for the same reason that an outside observer could not see in. “If I can’t see you, you can’t see me. It would be like being inside a silvery bubble,” explains Pendry. Would-be invisible men will have to figure out a way to cut out a visor, or perhaps decloak before accidentally walking into a wall.
Yet other metamaterial applications focus not on vanquishing what we see but on extending it. Padilla hopes to use engineered materials to investigate what he calls “the last unexplored region of the electromagnetic spectrum”—the T-ray, or terahertz, region—positioned between the infrared and microwave bands, at a frequency of a trillion cycles a second. Still largely inaccessible to our instruments, T-rays, if captured, could lead to innovative imaging and sensing technologies that hold enormous potential in biomedicine and security.
I knew everyone’s reaction would be that it could not be true. But physics sets no limits.
Already NASA scans T-rays to look for weaknesses in the foam insulation on the Space Shuttle. T-rays offer hope for improved detection of cancer, including skin cancers like melanoma, because they track molecular signatures of malignancy that are not easily seen with other types of scans. Padilla also envisions terahertz radiation playing a huge role in airport security. Not only can T-rays penetrate clothing to expose objects hidden underneath, but, because of their size and frequency, they can also reveal “whether that object is cheese or a plastic explosive,” something current scanning technologies simply cannot do. In the world of astronomy, the specific frequency of T-rays would allow scientists to observe the formation of stars.
In search of these riches, Padilla is building a metamaterial-based terahertz camera. Currently some companies use expensive, inefficient high-powered lasers to scan objects with T-rays, but Padilla’s scanner should quickly and cheaply collect images in a device only slightly larger than your digital camera. The metamaterial, partially made from gold, will absorb T-rays and convert them to heat, which the camera can then detect to form an image. Padilla has begun experimenting with custom materials and expects to have a functioning terahertz camera within three years.
And yet, for all this optimistic talk, metamaterials researchers are finding that their science is still young and the technological hurdles vast. The electromagnetic spectrum consists of an immense number of wavelengths. The details of how each artificial material works, and the kinds of engineered elements it requires, vary with the particular wavelength being manipulated. Most research done with microwaves cannot be generalized to the infrared or the visible parts of the spectrum. Engineering solutions aimed at modulating each part of the electromagnetic spectrum must be reasoned out from the beginning and pursued with a different set of materials and designs. Further complicating things, many applications will work only if you process many sets of wavelengths simultaneously. (An invisibility cloak that screens out only yellow would not be very useful.)
Furthermore, light moving through materials typically gets absorbed until, at some point, the energy of the radiation falls to zero, putting an end to its usefulness. How can metaengineers move radiation through artificial materials without snuffing it out? Moreover, some frequencies of light have such short wavelengths that building devices to interact with them would require pushing fabrication expertise past its current limits. Pendry is working on finding new synthetic materials that maintain the quirky properties of metamaterials without absorbing the precious light. “It’s bad enough in an ordinary lens, but it’s terrible for a perfect lens,” he says. Last October, Pendry published a paper in Science proposing new methods to allow engineers to harness light with minimal energy loss.
Scientists are inherently drawn to things once viewed as flat-out impossible.
Like technical hurdles of the past, though, this one too will probably soon yield to science. The reward for success is just too great to fail: Contemporary technology rests on a huge family of structures that interact with electromagnetic fields, from wireless modems to permanent magnets to lasers to tuners of all kinds. All these devices stand to be significantly improved—made cheaper, smaller, more capable, faster. Metamaterials seem perfect for building tiny spectrometers tuned to the presence of specific molecules, a function of interest to everyone from the health care industry to the Office of Homeland Security. It should be possible to build radiation shields that allow objects to sit in fields without being affected by them. Then normal surgical tools, like scalpels, could be used in an MRI machine without distorting the images on screen. The principles of metamaterials could be extended to control water waves (protecting oil rigs from sudden storms, for instance) and sound waves (creating perfectly quiet, private spots in the middle of a noisy office).
Beyond all this, scientists are inherently drawn to things once viewed as flat-out impossible. “I would give a talk on metamaterials,” remembers physicist Vladimir Shalaev of Purdue, “and people would come up to me and say, ‘Vlad, you have such a good reputation. Why would you want to throw it away by working in this field? Don’t you understand it cannot be true?’”
Now even his harshest critics have signed on for the show. Throw a rock into a pond and the ripples flow outward; toss a rock into a metamaterial pond and the ripples might flow inward, toward the point of impact. You could make the fish in that pond appear to swim in the sky. We aren’t there yet, but Pendry and his growing band of followers are building the bridge.