Invisibility Made Easier

In the past year, the media have been abuzz with talk of an exotic class of materials, called metamaterials, that could be used to make flat and distortion-free lenses, powerful microscopes, and even cloaking devices that make objects invisible. But versions of the materials suitable for practical applications have been difficult to make. Now researchers at Princeton University have demonstrated metamaterials that are both higher performing and much easier to manufacture, perhaps bringing these applications closer to reality.

"It's quite an important step," says Igor Smolyaninov, a research scientist at the University of Maryland who works with metamaterials. "It's much less expensive than anything else that people are doing."

Light passing from one ordinary material into another bends slightly--think of how a straight stick in water looks bent--but light passing into a metamaterial bends in the opposite direction. Metamaterials thus have what's called a negative index of refraction. A lens made from such a material wouldn't have to be curved. (It's the curvature of an ordinary lens that enables it to focus incoming light.) Metamaterials could also be used to route electromagnetic waves around an object, rendering it invisible. Already, researchers have demonstrated a cloaking device that makes objects invisible to microwaves, and others have created materials that negatively refract electromagnetic waves in the visible part of the electromagnetic spectrum. But until now, metamaterials have had to be patterned with intricate shapes smaller than the wavelength of light they're meant to manipulate. Consequently, materials that work with light of microscopic wavelengths, such as infrared and visible light, have been difficult to make. Because of the way they produce negative refraction, existing metamaterials have also had a strong tendency to absorb light, making them impractical for use in optics.

The materials developed at Princeton retain the property of negative refraction, yet they're much easier to make. Rather than requiring intricate structures, such as the split rings used in the microwave cloaking device, the materials can be made simply by stacking up extremely thin layers of semiconductor material. What's more, that stacking can be done by the same tools now used to make semiconductor materials for lasers used in telecommunications, says Claire Gmachl, the Princeton researcher who led the work. The new materials consist of alternating layers of indium gallium arsenide and aluminum indium arsenide, and they're tuned to work in the infrared region of the spectrum.

Like other metamaterials, the new materials affect light differently than ordinary materials do because they are made of structures significantly smaller than the wavelength of the light passing through them. In this case, however, it is the layers of semiconductors themselves that are thinner than the wavelength of light. Consequently, a wave passing through the material encounters multiple layers at once, responding to them as if they were a single material with properties quite unlike those of either semiconductor in isolation.

What makes the new materials different from previous metamaterials is that rather than changing two aspects of the way light moves, they change only one. If light is thought of as a wave, the wave front is perpendicular to the direction the light is moving. Imagine an ocean wave crashing ashore: it's moving in just one direction, but the wave front is a huge wall of water. Previous metamaterials changed the direction of light beams passing through them, and the wave front remained perpendicular to the direction of the beam. In the new materials, the light beam changes direction, but the wave fronts don't, giving the impression that they are slipping to the side rather than moving forward.

The overall effect on the direction of the light beam is the same as in the earlier metamaterial, but the new materials are simpler to create, and they absorb far less light, making them more attractive for use in optics.

The first application the Princeton researchers are developing is a flat lens for chemical-sensing devices, an application for which materials that work with infrared light are particularly well suited. Gmachl says that the current optical setups for such devices are bulky because they use conventional lenses. "The first application would be using that material to miniaturize optical setups" by replacing curved lenses with flat ones, she says.

Another early application could be in night-vision devices, which also work with infrared wavelengths. "For people who want to improve night-vision devices, this could be quite interesting," Smolyaninov says.

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