Metamaterials are grouped into a separate class of materials since their properties depend on the structure of the components being ordered in a special way, and can drastically differ from the properties of those components. Such metamaterials include, for example, synthetic dichroic materials consisting of isotropic components, where the asymmetric structure of the composite gives rise to the shape anisotropy. There are metamaterials with significantly increased permittivity and magnetic susceptibility and metamaterials, in which the efficiency of nonlinear effects increases by many orders of magnitude as compared to conventional materials. For example, the efficiency of giant Raman scattering may increase 106 times as compared to stimulated Raman scattering in the components; the efficiency of the second and third harmonic generations increases by orders of magnitude.

Although the possibility of controlling the structure of the components gives a new degree of freedom in designing their properties, the real revolution was produced by the works that demonstrated the possibility of creating metamaterials with properties that are not found in natural materials. The term "metamaterials" is most often used to refer to such materials. One of the best-known classes of metamaterials consists of metamaterials with negative refractive index, in which both permittivity and magnetic permeability are simultaneously negative. The existence of substances with simultaneously negative permittivity and permeability was theoretically justified in the work of V.G. Veselago published in 1967 [1]. As shown by the author, such substances are characterised by negative values of the refractive index, and many of their optical properties differ significantly from those of traditional materials. Natural materials with such properties have not yet been discovered. An experimental substance with negative refractive index in the radiofrequency range of electromagnetic waves was created in 1999 [2]. Currently, vast efforts have been made in the research and development of metamaterials with negative refractive index at optical wavelengths. All artificially created materials with simultaneously negative permittivity and permeability at optical wavelengths are composites containing metal and dielectric components.

A very promising class of metamaterials is formed by photonic crystals, in particular, resonant photonic crystals [3].