We recognize a material when we see it, though it's a fuzzy sort of intuitive concept. A piece of glass or plastic would serve as an example of a material, as might a section of copper wiring or part of an aluminum can. When we make the assertion that a substance is a material, we have certified that the substance has inherent physical properties from which we can assess how strong it is, how transparent or opaque it is, how well it conducts electricity, and so on. These properties are intrinsic, in the sense that if we cut or break the material into ever smaller pieces, all of the pieces will have identical physical properties.

This broad description provides a good working definition of a conventional material. However, we can imagine a much wider class of engineered artificial materials formed by carefully arranging other, smaller objects together in prescribed patterns. The concept of an artificial material has a long and well-established history in materials science and engineering (consider concrete used to construct buildings or carbon-fiber composites used in automobiles and aircraft). The application of this concept to optics (the study of visible light) or, more broadly, electromagnetism (the study of the entire spectrum of light, including infrared, ultraviolet, x-rays, radio waves, and microwaves) over the past decade has resulted in spectacular and profound advances.

There are at least two ways to make use of artificial materials. We can, for example, produce a material with properties unlike any found in nature. Alternatively, we can gain precise control over the properties of a material and vary them throughout a volume to create new devices. Two examples make clear the distinction: negative-index materials and invisibility cloaks.

When rays of light, visible to us or not, penetrate a material, they bend. The amount they bend—or refract—is determined by the material's index of refraction, an inherent physical property. In all conventional materials, the index of refraction is positive.  However, there is no fundamental restriction that prevents a material's index of refraction from being negative—they just don't happen to exist in nature. Russian physicist Victor Veselago predicted the possibility of negative refractive index materials in 1968. His theory showed that if such materials were found, nearly all of electromagnetism and optics would be turned on its head!

Veselago's prediction was realized in 2000, when negative refraction was experimentally confirmed in an artificial material, demonstrating the power of engineered materials to realize properties difficult or impossible to achieve with naturally occurring materials. To underscore their singularity, artificially structured materials are now generally referred to as metamaterials, indicating they have properties beyond those that can be found in naturally occurring materials.

 Metamaterials enable us to control light in entirely new ways. For centuries, optical devices have concentrated on sculpting the surfaces of simple materials to control light. The lenses in eyeglasses, cameras, or telescopes, for example, are usually made from glass or plastics, relying on the shape of the front and back surfaces to bend light rays. Light bends at the first surface, travels in a straight line through the material, and bends again at the second surface. With metamaterials, we can use every bit of the lens material—not just the surfaces—to steer and manipulate light in unprecedented ways.

As an example, our research team at Duke has designed an electromagnetic "invisibility cloak," a structure that, with some limitations, can render an object, and itself, invisible from detection over a narrow band of wavelengths. The cloak is designed so that light waves striking it are channeled around an object "hidden" inside (picture water flowing around a rock in a stream) and then emerge on the other side, giving the appearance of uninterrupted continuity.

But it gets much more exciting: In theory, we can build metamaterials so that they incorporate the means necessary to change or modulate their makeup, even after they have been constructed. The metamaterial elements that have commonly been employed for cloaks and other negative-index materials are actually composed of tiny electrical circuits, just as you might find in any electronic device. In principle, these metamaterial elements can have active components—like transistors or amplifiers—integrated into the design, allowing the material properties to be dynamically controlled. So, while the lens of today has its focal length and aperture fixed by geometry, the lens of tomorrow might be able to achieve a wide range of optical properties as the parameters of each constituent metamaterial circuit are tuned.

A single metamaterial lens may be able to mimic thousands of lenses by individually addressing each of the underlying metamaterial elements, much in the same way that we control the properties of each of the thousands of pixels in a flat-screen television to produce an image. The result might be a single eyeglass lens that could be programmed for any type of vision and, at the same time, serve as a magnifying and zoom lens.

As the science of metamaterials continues to draw from such fields as traditional optics, photonics, physics, materials engineering, electrical engineering, and nanotechnology, the opportunities continue to expand. If the past decade has been an indication, we have much to look forward to in the coming twenty-five years!

Smith, who is also a visiting professor in the physics department at Imperial College, London, received his B.S. and Ph.D. in physics from the University of California at San Diego. While a postdoctoral researcher there, he demonstrated the first negative-index material, leading to the development of the field of metamaterials. His work was twice named among the "Top Ten Breakthroughs" by Science magazine, in 2004 and 2006, while metamaterial cloaking was selected as one of the "Top Ten Emerging Technologies" by Technology Review in 2007. Smith was named one of the "Scientific American 50" researchers and policymakers in 2006.