In certain periodic dielectric structures, there arises a phenomenon called a photonic band gap (PBG): a range of frequencies in which light cannot propagate. A PBG is a purely classical phenomenon, arising only from Maxwell's equations.

While 1D PBGs were known (by other names) for many decades, 2D and 3D PBGs date only from 1987. Structures exhibiting a PBG are sometimes called "photonic crystals."