The field of metamaterials arose as a combination of advances in fabrication capabilities and developments in the physical understanding of how matter interacts with electromagnetic waves. Metamaterial advances span the electromagnetic spectrum, with examples being more common at lower (e.g., microwave) frequencies. The microwave or x-band regime has proven to be a good testbed for the first generation of metamaterials; however, recently, we have seen optical-range metamaterials emerging as well. The discovery of these more complex material–wave interactions has come about when nano-fabrication techniques allow structures to be formed with dimensions much smaller than the wavelength of (e.g., visible) light. We can now, in principle, engineer material properties as we see fit, through arrangements of meta-atomic structures, or small geometrically designed collections of atoms that have very specific responses to incident radiation. Losses in these materials will remain a major challenge, as we will see.
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