Photonic crystals (PCs) are optical nanostructures which their electric permittivity is periodic in one, two, or three dimensions. This periodic permittivity affects photons in the same manner as periodic potential affects electrons in ionic crystals. As a result, electromagnetic waves in these materials are in the form of Bloch waves. There are some finite and continuous frequency bands in which electromagnetic waves cannot propagate in PCs. Existence of these bands which are called photonic gaps, is the main property of PCs.Lord Rayleigh, the English physicist, observed photonic gaps in a one-dimensional PC for the first time in 1887, but until a century later, when Yablono vitch predicted the existence of photonic gaps in three-dimensional PCs, there were no uses of them.