Negative-index metamaterial

An engineered composite with an effective refractive index below zero across a frequency range. In that range, electromagnetic waves refract opposite the direction expected in ordinary materials.

Refractive index: A quantity describing how electromagnetic waves propagate through a material relative to vacuum. A negative effective value defines this material's distinctive wave behavior.
Veselago medium: A hypothetical medium with simultaneously negative permittivity and permeability, proposed by Victor Veselago. It supplied the theoretical model later realized with metamaterial structures.
Perfect lens: An ideal lens that recovers evanescent as well as propagating waves to form a subwavelength image. Negative refraction was proposed as a way to amplify evanescent waves for finer imaging.
Positive-index material: A material whose refractive index is positive over the frequency range of interest. Its refraction provides the usual directional behavior against which negative refraction is defined.
Snell's law: An equation relating the angles of incidence and refraction to the refractive indices of two media. With a negative index, the transmitted ray bends to the opposite side of the normal.
Victor Veselago: A Soviet physicist who analyzed electromagnetic wave propagation in materials with negative permittivity and permeability. His 1968 analysis predicted the unusual refraction underlying this concept.
Superlens: A lens that uses a material response to recover some subwavelength image detail beyond the diffraction limit. Negative-index behavior can support super-resolution imaging in suitable designs.
Photonic crystal: A periodic optical structure whose band structure controls the propagation of light. Photonic crystals can show unusual refraction without being negative-index composites.
Permittivity: A measure of how a material responds to an electric field. Negative effective permittivity is one of the parameters used to obtain a negative index.
Split-ring resonator: A resonant conducting structure designed to produce a strong magnetic response at selected frequencies. Split-ring arrays enabled artificial magnetic responses in early negative-index designs.

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Photonic metamaterialBroader topic: It is a prominent example of optical behavior engineered through subwavelength structure.