The photonic bandgap is an area within a photonic crystal where a wavelength of light is forbidden. This is due to the energy required for the light to pass though, what can be considered in terms of a diffraction grating, as being unfavourable. The energy is defracted along a different path and not simply placed into reverse. Opals are photonic crystals whch have a three dimensional dielectric structure with lattice parameters in the micron range on the order of the visible wavelength of light.

The forbidden gap has important quantum optical consequences due to the inhibition of spontaneous emissions which may lead to novel optical devices such as very fine focus lasers which are also very cheap to produce.

While it currently outside of the remit of this project, there have been a number of papers on the emission of light from photonic structures.