Photonic crystal

Summary

A photonic crystal is a material whose permittivity varies periodically in space, with a period comparable to the wavelength of light. Light in it behaves like electrons in a crystal: its allowed frequencies form bands, separated in good cases by photonic band gaps. In a band gap no light of that frequency can travel through the crystal, in any direction for a complete gap. Photonic crystals are used to make mirrors, waveguides with sharp bends, low-loss fibers and small optical cavities.