A photonic crystal is an optical material that has a periodic structure on the scale of the wavelength of light. This periodicity creates a photonic band gap, which is a range of wavelengths (or frequencies) over which light cannot propagate through the material. Just as a semiconductor crystal can control the flow of electrons, a photonic crystal can control the flow of photons (light).
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A structure with spatially periodic dielectric response, giving optical bands and, under suitable conditions, forbidden frequency intervals.