A distinctive feature of photonic crystals (PCs) is spatially periodic variation in refractive index. Depending on the number of spatial directions with the refractive index periodically changing along them, photonic crystals are called one-dimensional, two-dimensional and three-dimensional, or abbreviated as 1D PC, 2D PC and 3D PC (D meaning dimension), respectively. See the Figure for the schematic structure of 2D and 3D PCs.

The most striking feature of photonic crystals is that in 3D PCs with quite a great refractive index there exist certain regions of the spectrum, known as full photonic band gaps (PBG): emission or propagation of photons with energies lying within the PBG is impossible in PCs. In particular, the radiation with spectrum within PBG does not penetrate from the outside into the PC, cannot exist inside it and is totally reflected from the boundary. Prohibition is broken only in the case of structural defects or the small size of the PC. At the same time, linear defects, which were produced deliberately, are the waveguides with low bending loss (to micron radius of curvature), point defects are tiny cavities. The practical implementation of the 3D PC potential based on controllability of light (photon) beam performance is just at the beginning of the road. It is difficult due to the lack of effective methods of creating high quality 3D PCs, targeted and controllable ways to form such local heterogeneities, linear and point defects, as well as methods to interface with other photonic and electronic devices.

More significant progress has been made towards the practical application of 2D PCs, which are usually used in the form of planar (film) photonic crystals or in the form of photonic crystal fibres (PCF) (see for more detail in the relevant articles).

A PCF is a two-dimensional structure with a defect in the central part extended perpendicularly. As a fundamentally new type of optical fibres, PCFs provide options to transport light waves and control light signals, which are not available to other types of optic fibres.

1-dimensional PCs (1D PCs) are a multi-layered structure of alternating layers with different refractive indices. In classical optics, long before the term photonic crystal appeared, it was well known that in such periodic structures, the nature of light waves changes significantly due to interference and diffraction. For example, multilayer reflective coatings have been in wide use in the production of dielectric mirrors and thin-film interference filters for a long time; and volume Bragg gratings used as spectral selectors and filters. After the term PC became common, such layered media, where the refraction index varies periodically along one direction, were referred to one-dimensional photonic crystal class. When light is incident perpendicularly, the spectral dependence of reflectivity on multilayer coatings is the so-called Bragg table - at some wavelengths the reflective index approaches one rapidly as the number of layers increases. The light waves that get within the spectral range, as shown in Fig. B with an arrow, reflect almost all from the periodic structure. In PC terminology, this wavelength region and the respective range of photon energies (or energy band) is a gap for light waves going perpendicularly to the layers.

The PC practical application potential is huge because of the unique controllability of the photons and there is still a long way to go to utilize it to the full. There is no doubt that in the years to come there will be new devices and structural elements to offer, which may be fundamentally different from those used or being developed today.

The great outlook for PC application in photonics was realised after E. Yablonovich published his article, proposing to use PCs with full PBG to control the spontaneous emission spectrum.

The photonic devices to be expected in the near future include:

- Low-threshold ultra size PC lasers;

- High-brightness PC LEDs with controllable emission spectrum;

- Subminiature PC waveguides with micron bend radius;

- Photonic integrated circuits with a high degree of integration based on planar PCs;

- Miniature PC-spectral filters, including tunable ones;

- Optical random-access storage PC devices;

- Optical signal PC-processing devices;

- Means to deliver high-power laser radiation based on hollow PCF.

The most attractive, but also the most challenging to implement the use of three-dimensional PCs is the creation of very large volume integrated sets of photonic and electronic devices to process information.

Other possible applications of three-dimensional photonic crystals include artificial opal jewellery fabrication.

Photonic crystals are found in natural environment, adding colour shades to the world around us. Thus, pearl coating of mollusk shells, such as abalone (Haliotidae), has a 1D PC structure, sea-mouse antennae and polychaetes bristles are 2D PCs, and natural semi-precious stones, opals and the wings of African swallowtails (Papilio ulysses) are natural three-dimensional photonic crystals.