Photonic Band Gaps

Abstract

Photonic band gaps are ranges of frequency within which electromagnetic propagation is completely forbidden. They are present in certain materials which possess a periodicity of permittivity at the wavelength scale. Materials with these extreme properties are not known to occur naturally, and. at the optical wavelength scale, require fabrication methods at the current limits of technological feasibility. Such a photonic crystal provides a lossless barrier to propagation, and can suppress the emission of a photon by a decaying atom if the frequency of the emitted photon lies within the gap. A preferred propagation route, or mode, can be specified by designed defects within the photonic crystal; thus it is expected that I photonic crystals can provide a means whereby spontaneous emission is controlled in active media, and that all the spontaneously emitted light enters a single mode, resulting in an ideal zero-threshold laser. More generally, the photonic density of states is altered in these materials, and spontaneous emission can be enhanced or suppressed, as required. Other applications include novel all-angle reflectors, narrow-band filters, resonators, waveguides, and delay lines. When the fabrication problems for optical photonic crystals have been conquered, wavelength-scale periodic media will form an essential functions in a large range of optoelectronic systems.