Metamaterial
In electromagnetism (covering areas like optics and photonics), a meta material (or metamaterial) is an object that gains its (electromagnetic) material properties from its structure rather than inheriting them directly from the materials it is composed of. This term is particularly used when the resulting properties have not been found for naturally formed substances.
Metamaterials are often made up of a collection of objects whose separation is much less than the wavelength of light passing through it. Using a wider definition, the most common example of metamaterial would be an optical medium such as glass; here the objects would be the individual atoms or molecules making up the substance.
However, it is possible to construct arrays of current-conducting elements (such as loops of wire) that have a magnetic response at microwave frequencies. When electromagnetic radiation passes the wire loops, the magnetic field could induce an electric current, thus producing magnetic response.
Negative refractive index
Very nearly all materials encountered in optics, such as glass or water, have positive values for both <math>\epsilon<math> and <math>\mu<math>. However, many metals (such as silver and gold) have negative <math>\epsilon<math> at visible wavelengths. A material having either (but not both) <math>\epsilon<math> or <math>\mu<math> negative is opaque to electromagnetic radiation (see surface plasmon for more details).
Although the optical properties of a transparent material are fully specified by the parameters <math>\epsilon<math> and <math>\mu<math>, in practice the refractive index <math>N<math> is often used. <math>N<math> may be determined from <math>N=\sqrt{\epsilon\mu}<math>. All known transparent materials possess a positive index because <math>\epsilon<math> and <math>\mu<math> are both positive.
However, some engineered metamaterials have <math>\epsilon<0<math> and <math>\mu<0<math>;
because the product <math>\epsilon\mu<math> is positive, <math>N<math> is real.
Under such circumstances, it is necessary to take the negative square root for <math>N<math>. Physicist Victor Veselago proved that such substances are transparent to light.
Metamaterials with negative <math>N<math> have numerous startling properties:
- Snell's law (<math> N_1\sin\theta_1=N_2\sin\theta_2<math>) still applies; but rays are refracted away from the normal on entering the material
- The Doppler shift is reversed (that is, a light source moving toward an observer appears to reduce its frequency)
- Cerenkov radiation points the other way
- group velocity is antiparallel to phase velocity (as opposed to parallel for normal materials)
One common metamaterial is the Swiss roll.
