How Metamaterials Work
How do metamaterials work? Is there a simple qualitative answer?
The answer must begin with a short discussion of why natural materials exhibit the properties they do. Knowing this, it is straightforward to see that metamaterials mimic the behavior of natural materials, but in ways that allow people to engineer their properties, make the properties stronger, and achieve property values not observed in natural materials.
When an electromagnetic wave propagates through a natural material (i.e. glass, ceramic, air, etc.), the electric and magnetic fields put forces on the electrons and protons that push those particles around. When a charged particle moves, the acceleration of charge produces a weak secondary electromagnetic wave that radiates away.
These secondary waves have the same frequency as the original applied wave but out of phase, so they interfere with the applied wave. When this interference is averaged over many atoms, the applied wave appears to slow down and/or alter its amplitude. These effects are described using permittivity and permeability. The permittivity quantifies the effects of the electric fields while permeability quantifies the effects of the magnetic fields.
Metamaterials are typically composed of an array of metallic elements that are spaced at a distance smaller than a wavelength. An applied electromagnetic wave induces currents in the elements. The currents in the elements make the elements act like tiny antennas and each radiates a secondary wave. These secondary waves have the same frequency as the original applied wave but are out of phase, so they interfere with the applied wave. When this interference is averaged over many elements, the applied wave appears to slow down and/or alter its amplitude, exactly like happens in natural materials. Therefore, the metamaterial itself exhibits an effective permittivity and permeability.
Many argue that the properties of a metamaterial are artificial and not real or physical. However, the same argument could be made for natural materials because they exhibit their properties due to the same physical mechanism.
For both natural materials and metamaterials, the applied wave does not actually change speed or alter its amplitude. It is the interference of the applied wave with all the secondary waves that makes it look as if this is happening when an average is taken and we pretend it is still just the applied wave. I argue the effective properties of metamaterials are just as real as the properties of natural materials.