Photonic metamaterials are artificial optical materials which are based on the periodic arrangement of functional building blocks (“artificial atoms”). The corresponding lattice constants are much smaller than the relevant wavelength of the incident electromagnetic radiation. Thus, the light field "sees" an effective homogeneous material which can be characterized by an effective permeability μ and an effective permittivity ε.
By properly designing the "artificial atoms", one can create photonic metamaterials with optical properties which have no analogue among natural substances. In particular, photonic metamaterials allow for a magnetic response (μ≠1) at optical frequencies and even a negative index of refraction (n<0). Thus, photonic metamaterials open a whole new chapter of photonics connected with novel concepts.

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SEM image of Split Ring Resonators as an intensively studied example of photonic metamaterials.