Reverse Charge of the Light Brigade

Nanowire metamaterial. (Photo by Xiang Zhang.)

Light interacts with glass, water and other transparent materials in long-understood ways that define the capabilities of traditional optical devices. But Professor Xiang Zhang’s lab is engineering materials with fundamentally new optical properties that could enable far more powerful microscopes and microchips, denser optical storage, and even—disclaimers in place—the very beginnings of an invisibility shield that camouflages objects by bending light around them.

In regular transparent materials, light is briefly absorbed and re-emitted by the atoms along its path, which form a sort of photonic bucket brigade. This slows the light down, which is how lenses work; when a light wave enters glass at an angle, its inside edge slows down first, bending the light toward the glass.

What makes the new materials special is that they refract negatively, reversing the light instead of just bending it down. Consequently, they function like both a lens and a mirror. Zhang’s co-researcher Shuang Zhang (no relation) uses the analogy of a rock dropping into a pond.  In a normal pond, you see a slowly expanding circle of waves, a wave packet, composed of faster moving individual ripples that form and decay as the larger group moves outward. In a hypothetical negative-refracting pond, the wave packet would still expand slowly outward, but the individual waves would ripple toward the center. In a hypothetical glass of negative-refracting water, a leaning straw would appear as its mirror image under the surface.

Although such materials have been hypothesized since the 1960s, some scientists argued that they were impossible. Researchers at UC San Diego ended the argument in 2000, using printed copper traces and thin wire. But these early examples worked only with microwaves, which can’t penetrate or resolve as much detail as visible light. Recently, with novel designs empowered by nanofabrication technologies, Zhang and his team have created two negative-refracting materials that work close to or at visible red, inching towards full, living color.

The simpler of Zhang’s two materials consists of silver nanowires embedded straight and parallel through a slice of polycrystalline sapphire. Angle a beam of light onto its surface, and it behaves as you might expect: light that’s polarized parallel to the wires refracts through normally, while light that’s polarized perpendicularly angles backwards, as if the silver nanowires were tiny mirrors. This behavior holds over a wide range of angles and frequencies that include visible red light.

The other material is a waffle-shaped “fishnet” made of alternating layers of silver and magnesium fluoride. The layers form a 3-D array of nanoscale capacitors and inductors, which act as optical circuits and influence neighboring circuits in a way that propagates a light wave just like any transparent medium would—except that the photonic bucket brigade runs in reverse. Says Shuang Zhang, “It works like a transmission line but is designed in such a way that the wave front runs in the opposite direction.” The material works with near-infrared light, a wavelength commonly used for long-distance optical communications.

Both are known as “metamaterials” because their engineered structure gives them properties not found in natural materials. Both also exhibit negative refraction, but only the fishnet truly has a refraction index that measures less than zero.

Fishnet metamaterial. (Photo by Xiang Zhang.)

As Xiang Zhang explains, the materials’ unusual properties make them “more than just a scientific curiosity.” Because negative-refraction metamaterial reverses light’s direction internally, a flat piece creates an image, like a conventional round lens; but the image is far sharper, able to capture super-fine details that are smaller than the half-wavelength limit of conventional optics. Positioned close to its subject, the metamaterial catches scattered light that other lenses are incapable of capturing and that in turn can be magnified using traditional optics.

A microscope with these materials would combine the best aspects of optical microscopes, which can capture living processes and movement, and electron and atomic force microscopes, which can visualize objects smaller than optical scale but work only for still or dead subjects, because they build their images line-by-line like a scanner.

Metamaterials engineered to have very high refraction indexes can also be made into more powerful lenses for optical lithography and computer hardware. Using these lenses, lasers could etch and read from surfaces in much finer detail, permitting huge power and speed increases in microprocessors and similarly higher densities for optical media such as DVDs.

Because these metamaterials can bend light, they might some day camouflage things by steering light around them from all angles. Time magazine’s Best Inventions of 2008 referred to the metamaterials as “the invisibility cloak” used by Harry Potter. But Shuang Zhang points out that today’s materials work for only a narrow wavelength range and lose light quickly. “It might cloak red light but not green,” he explains, “and the object would have to be so tiny, micrometer scale, that you wouldn’t be able to see it anyway.” Potter’s fantasy cloak is far from reality, Zhang says. “People are making progress towards it, but there is still a long way to go.”