What is a photonic crystal?
A photonic crystal is a periodic structure of materials with differing refractive indices. Therefore, the optical multilayer films widely used as anti-reflection films etc. for the surface of television monitors, glasses and other optical components can be considered to be one dimensional photonic crystals. However, generally the term photonic crystal refers to two dimensional and three dimensional structures. The pitch of the structure is usually chosen such that it is half the wavelength of the light for which the element is designed for. Typically, photonic crystals used in the visible optical region are designed and fabricated so that the period is about 300nm.
Like optical multilayer films, photonic crystals are used to control optical transmission, reflection and refraction characteristics. Using 2D and 3D photonic crystals it is possible to control the propagation of light at arbitrary angles of incidence and not only the light normally incident as is the case for conventional optical films. Further, using photonic crystals, it is possible to achieve optical functionality not possible using conventional optical materials.