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Photonic structures control light through the spatial arrangement of dielectric material. Waveguides confine transverse modes; periodic photonic crystals combine interference and scattering to produce a photonic band structure ${\omega }_n(\mathbf{k})$. A photonic band gap excludes propagating modes in specified directions and polarizations. A one-dimensional stop band is not automatically a complete three-dimensional gap.

A simple example is a distributed Bragg reflector with alternating high- and low-index layers. At normal incidence, the quarter-wave choicelets successive interface reflections reinforce near the design wavelength. A departure from periodicity can create a localized defect mode inside the stop band. An isolated defect forms a cavity; a line defect can guide light through otherwise forbidden bulk propagation. Finite structures leak and real materials absorb, so ideal band arguments do not ensure infinite confinement.

Changing the photonic density of states can suppress or enhance spontaneous emission and control how emitters couple to selected modes. This is distinct from electronic band-gap engineering: it is the electromagnetic modes that are being structured. A complete electromagnetic gap overlapping an emitter's transition is the ideal mechanism proposed for suppressing otherwise allowed spontaneous emission.^[ref]

A photonic crystal has a spatially periodic dielectric response on the scale of the optical wavelength. For a nonmagnetic, lossless, nondispersive dielectric, the source-free Maxwell equations can be written as the eigenproblemThe Bloch theorem gives ${\mathbf{H}}_{n\mathbf{k}}=e^{i\mathbf{k}\cdot \mathbf{r}}{\mathbf{u}}_{n\mathbf{k}}(\mathbf{r})$ with periodic $\mathbf{u}$. The allowed frequencies form photonic band structures. Coupling of counterpropagating waves near a Brillouin zone boundary opens a stop band. A one-dimensional dielectric multilayer is a simple example; two-dimensional hole arrays and three-dimensional dielectric networks allow more extensive control.

A complete photonic band gap excludes bulk propagating modes for every direction and polarization in a frequency interval. Periodicity alone does not guarantee such a gap: dimensionality, symmetry and refractive-index contrast matter. A missing or altered element can support a localized defect state in the gap, making a cavity; a line defect can guide light. Finite crystals have leakage and wavelength-dependent transmission rather than an infinitely sharp exclusion. Applications include selective mirrors, optical filters, small resonators and integrated waveguides.^[ref]