A structure with spatially periodic dielectric response, giving optical bands and, under suitable conditions, forbidden frequency intervals.
The frequencies of electromagnetic Bloch modes form allowed bands in a periodic dielectric. Their gaps and curvature determine forbidden propagation, confinement and group velocity.
A frequency interval without bulk propagating electromagnetic modes in the specified directions and polarizations. A complete gap excludes all directions and polarizations; a one-dimensional stop band need not do so.
A local departure from periodicity can support a discrete mode inside a bulk photonic band gap. An isolated defect makes a cavity; a line of defects may guide light. Finite surrounding crystals allow leakage.
An ordered array of colloidal building blocks whose optical-scale periodicity gives Bragg reflection and photonic stop bands. Particle size and refractive-index contrast control the response; defects and stacking disorder can broaden it.
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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).