metamaterial, an artificially structured material that exhibits extraordinary electromagnetic properties not available or not easily obtainable in nature. Since the early 2000s, metamaterials have emerged as a rapidly growing interdisciplinary area, involving physics, electrical engineering, materials science, optics, and nanoscience. The properties of metamaterials are tailored by manipulating their internal physical structure. This makes them remarkably different from natural materials, whose properties are mainly determined by their chemical constituents and bonds. The primary reason for the intensive interest in metamaterials is their unusual effect on light propagating through them.
Metamaterials consist of periodically or randomly distributed artificial structures that have a size and spacing much smaller than the wavelengths of incoming electromagnetic radiation. Consequently, the microscopic details of these individual structures cannot be resolved by the wave. For example, it is difficult to view the fine features of metamaterials that operate at optical wavelengths with visible light, and shorter-wavelength electromagnetic radiation, such as an X-ray, is needed to image and scan them. Researchers can approximate the assemblage of inhomogeneous individual structures as a continuous substance and define their effective material properties at the macroscopic level. Essentially, each artificial structure functions as an atom or a molecule functions in normal materials. However, when subjected to regulated interactions with electromagnetic radiation, the structures give rise to entirely extraordinary properties. (Some naturally occurring materials such as opal and vanadium oxide do exhibit unusual properties when they interact with electromagnetic radiation and have been called “natural metamaterials.” However, metamaterials are most often known as artificially occurring materials.)
An example of such extraordinary properties can be seen in electric permittivity (ε) and magnetic permeability (μ), two fundamental parameters that characterize the electromagnetic properties of a medium. These two parameters can be modified, respectively, in structures known as metallic wire arrays and split-ring resonators (SRRs), proposed by English physicist John Pendry in the 1990s and now widely adopted. By adjusting the spacing and size of the elements in metallic wire arrays, a material’s electric permittivity (a measure of the tendency of the electric charge within the material to distort in the presence of an electric field) can be “tuned” to a desired value (negative, zero, or positive) at a certain wavelength. Metallic SRRs consist of one or two rings or squares with a gap in them that can be used to engineer a material’s magnetic permeability (the tendency of a magnetic field to arise in the material in response to an external magnetic field). When an SSR is placed in an external magnetic field that is oscillating at the SSR’s resonant frequency, electric current flows around the ring, inducing a tiny magnetic effect known as the magnetic dipole moment. The magnetic dipole moment induced in the SRR can be adjusted to be either in or out of phase with the external oscillating field, leading to either a positive or a negative magnetic permeability. In this way, artificial magnetism can be achieved even if the metal used to construct the SRR is nonmagnetic.
By combining metallic wire arrays and SRRs in such a manner that both ε and μ are negative, materials can be created with a negative refractive index. Refractive index is a measure of the bending of a ray of light when passing from one medium into another (for example, from air into water or from one layer of glass into another). In normal refraction with positive-index materials, light entering the second medium continues past the normal (a line perpendicular to the interface between the two media), but it is bent either toward or away from the normal depending on its angle of incidence (the angle at which it propagates in the first medium with respect to the normal) as well as on the difference in refractive index between the two media. However, when light passes from a positive-index medium to a negative-index medium, the light is refracted on the same side of the normal as the incident light. In other words, light is bent “negatively” at the interface between the two media; that is, negative refraction takes place.
Negative-index materials do not exist in nature, but according to theoretical studies conducted by Russian physicist Victor G. Veselago in 1968, they were anticipated to exhibit many exotic phenomena, including negative refraction. In 2001 negative refraction was first experimentally demonstrated by American physicist Robert Shelby and his colleagues at microwave wavelengths, and the phenomenon was subsequently extended to optical wavelengths. Other fundamental phenomena, such as Cherenkov radiation and the Doppler effect, are also reversed in negative-index materials.
In addition to electric permittivity, magnetic permeability, and refractive index, engineers can manipulate the anisotropy, chirality, and nonlinearity of a metamaterial. Anisotropic metamaterials are organized so that their properties vary with direction. Some composites of metals and dielectrics exhibit extremely large anisotropy, which allows for negative refraction and new imaging systems, such as superlenses (see below). Chiral metamaterials have a handedness; that is, they cannot be superimposed onto their mirror image. Such metamaterials have an effective chirality parameter κ that is nonzero. A sufficiently large κ can lead to a negative refractive index for one direction of circularly polarized light, even when ε and μ are not simultaneously negative. Nonlinear metamaterials have properties that depend on the intensity of the incoming wave. Such metamaterials can lead to novel tunable materials or produce unusual conditions, such as doubling the frequency of the incoming wave.
The unprecedented material properties provided by metamaterials allow for novel control of the propagation of light, which has led to the rapid growth of a new field known as transformation optics. In transformation optics, a metamaterial with varying values of permittivity and permeability is constructed such that light takes a specific desired path. One of the most remarkable designs in transformation optics is the invisibility cloak. Light smoothly wraps around the cloak without introducing any scattered light, thus creating a virtual empty space inside the cloak where an object becomes invisible. Such a cloak was first demonstrated at microwave frequencies by engineer David Schurig and colleagues in 2006.
Owing to negative refraction, a flat slab of negative-index material can function as a lens to bring light radiating from a point source to a perfect focus. This metamaterial is called a superlens, because by amplifying the decaying evanescent waves that carry the fine features of an object, its imaging resolution does not suffer from the diffraction limit of conventional optical microscopes. In 2004, electrical engineers Anthony Grbic and George Eleftheriades built a superlens that functioned at microwave wavelengths, and in 2005, Xiang Zhang and colleagues experimentally demonstrated a superlens at optical wavelengths with a resolution three times better than the traditional diffraction limit.
The concepts of metamaterials and transformation optics have been applied not only to the manipulation of electromagnetic waves but also to acoustic, mechanic, thermal, and even quantum mechanical systems. Such applications have included the creation of a negative effective mass density and negative effective modulus, an acoustic “hyperlens” with resolution greater than the diffraction limit of sound waves, and an invisibility cloak for thermal flows.
