Electromagnetic Metamaterials
Electromagnetic metamaterials are artificially structured materials that are designed to interact with and control electromagnetic waves. Electromagnetic waves might be any type of wave in the electromagnetic spectrum (shown here on the figure to the right). Most of us are familiar with light waves in the visible, which occupy a rather small portion of the electromagnetic spectrum. Visible light waves have wavelengths from 400 to 700 nanometers (a nanometer is one-billionth of a meter), yet electromagnetic waves can have wavelengths of thousands of kilometers to trillionths of a meter!
When light (or electromagnetic) waves enter a material, the electric and magnetic fields of the wave cause electrons within the material to move around. This exchange of electromagnetic energy with the atoms and molecules of a material is the means by which materials can be used to control and manipulate light waves, forming the basis for electromagnetic devices.
The size and typical spacing of atoms within a material are on the order of angstroms, or tenths of one nanometer. That means that visible light waves, which are hundreds of nanometers in size, or longer wavelength waves cannot even come close to resolving the atomic structure. Although we know materials are formed from collections of atoms, we cannot see the individual atoms because the light we perceive is so much larger than the atomic scale. So, we are able to approximate the discrete atoms and molecules of a material as a continuous substance, whose properties derive not only from the individual atoms and molecules, but also their interactions.
We can easily come up with examples of optical devices, based on our experience with visible light. The lenses in telescopes, microscopes or eye glasses, for example, are simply pieces of plastic or glass that take rays of light and cause them to converge or diverge. The properties of a lens are related to the material of which it is made, as well as its shape. Optical fibers and waveguides represent other classes of optical devices, in which the material is used to guide light from one point to another, like water passing through a pipe. Optical fibers are formed by 'pulling' carefully designed and optimized combinations of glasses, and are used to transmit light over surprisingly large distances.
The quality and diversity of optical devices is, at least in part, determined by the available range of electromagnetic properties of the materials used to make the devices. There are interesting opportunities here, because existing materials display only a subset of the electromagnetic properties that are theoretically available. Since we know that, ultimately, materials consist of atoms and molecules, it would seem reasonable to try to expand the available range of material properties by adjusting the composition of materials at the molecular level using chemistry. But there is another way: We can broaden our definition of a material. In effect, we can "fool" light by taking any arrangement of objects and assembling them into some sort of structure. If the size and spacing of the objects are much smaller than the wavelength of light, then the light will not be able to resolve the difference between our collection of objects and an actual material. What is the advantage? As it turns out, material properties obtained by engineering the geometry of macroscopic objects can extend well beyond what is obtainable by chemical synthesis. Consequently, a structured material is now often referred to as a metamaterial, since its electromagnetic properties are often beyond those of any known naturally occurring materials.
Just to avoid any confusion, it is clear that all materials are composed of atoms and molecules--we can't get around that. However, the metamaterials concept allows us to avoid the techniques of chemical synthesis, and arrive at new electromagnetic materials by changing the geometry of other objects. We no longer need to think about reaction dynamics, but rather how to design the geometry of metamaterial elements so that a composite formed from these elements will have desired properties.