The periodic arrangement of ions on a lattice gives rise to the energy band structure in semiconductors.
Energy bands control the motion of charge carriers through the crystal.
Similarly, in a photonic crystal, the periodic arrangement of refractive index variation, controls how photons are able to move through the crystal. Photons react to the refractive index contrast in an analogous manner to the way electrons react when confronted with a periodic potential of ions. Each results in a range of allowed energies and a band structure characterised by an energy gap or photonic band gap. A band gap forms when the electron wavelength is comparable to the inter-atomic spacing. Photonic band gaps were first predicted in 1987 by two physicists working independently.