 Solution

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]