Photonic band structure

The frequencies ${\omega }_n(\mathbf{k})$ 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.