Wiktionary has a category on Metamaterials.

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Metamaterials are artificial materials engineered to provide properties which may not be readily available in nature. These materials employ the inclusion of small inhomogeneities to enact effective macroscopic behavior. The primary research in metamaterials investigates materials with negative refractive index. Although the first metamaterials were electromagnetic, photonic, acoustic and seismic metamaterials are also areas of active research.