Photonic Crystals are used to controlling the propagation of light. The field of photonic crystals is a marriage of solid state physics and electromagnetism. Crystal structures are citizens of solid-state physics, but in photonic crystals the electrons are replaced by electromagnetic waves.

Photonic Crystals allow the complete control of light dependent on its structure. A crystal is a periodic arrangement of atoms or molecules. A crystal therefore presents a periodic potential to an electron propagating through it. Unlike conventional crystals, a photonic crystal is a lattice of macroscopic dielectric media instead of atoms. For the seamouse, this is the periodic hexagonal structure shown above which exists throughout the sea mouse's iridescent hairs. If the refractive indices of the two different mediums are great enough(the hair and surrounding water), the scattering off the different surfaces can produce the same phenomena for photons as the atomic potential does for electrons. So far, photonic crystals have been artificially produced by using optical devices such as a dielectric mirror, or a stack of dielectric interfaces with alternating refractive index. Hence the discovery of the sea mouse's photonic properties is of particular interest since it is a natural phenomenon with much research potential and perhaps even applications in opto-electronics.

As a normal wave is incident on the structure it may diffract and scatter through the structure but it will also split into forward and reverse waves. As the forward wave continues into the crystal it will encounter more and more obstacles that cause more diffraction, scattering and forward and reverse waves which creates the modal patterns that we have visualised.

The periodic spacing of the hexagonal structure may lead to the forward and reverse waves inside the crystal to destructively interfere with each other. This generates the photonic band gap ensuring that the radiation cannot propagate through the crystal. A photonic band gap covers a range of frequencies where any incident wave arriving onto the crystal will be reflected rather than transmitted. When a photonic crystal reflects light of any polarization incident at any angle, the crystal is said to have a complete photonic band gap. In such a crystal, no light modes can propagate if they have a frequency within that range. A simple dielectric mirror cannot have a complete band gap, because scattering occurs along one axis. A material with a complete photonic band gap must be periodic along all three axes. The seamouse hair structure is 3 dimensionally symmetrical so has a complete band gap at certain wavelengths. Quantitatively, the band gap is due to Bragg diffraction through the different refractive indices of the strucutre, where waves are forbidden to propagate in certain directions with certain energies. Bragg diffraction will only occur when the wavelength is an integer multiple of the diffraction spacing:

Unable to go through the crystal the radiation is reflected. For a full band gap to exist every direction should lead to reflection regardless of polarisation. A fully three dimensional band gap will reflect all incident radiation upon it no matter from which direction it comes from for a given frequency, i.e. there are no allowed modes. This is why the seamouse's hairs appear iridescent, due to this reflection. The different wavelengths are all reflected, although the band gap for each wavelength is slightly different meaning the separate colours will be reflected at slightly different points separating the white incident light into many colours when it is reflected from the structure.

Here's an electron micrograph of a cross-section from a 3-dimensional photonic crystal with the same structure as the seamouse. State of the art semiconductor fabrication was used to make this structure yet it occurs naturally on the sea mouse.

This is accomplished by nanofabrication of a structure which has 3-dimensional periodicity in the dielectric constant. These structures make good reflectors since photons with energies within the band gap are not allowed and so will not propagate inside the crystal. Photonic crystals are interesting for optoelectronic devices because they make it possible for photons to be controlled and confined in small structures (on the scale of one wavelength cubed). Our primary interest is in application of photonic crystal structures to novel light-emitting devices such as light emitting diodes (LED's) and semiconductor lasers. These devices should be able to achieve greater efficiency, smaller size and greater modulation speeds.

Our control over materials includes their mechanical and electrical properties. In the last two decades a new frontier emerged to control the optical properties of materials. The objective has been to engineer novel artificial materials for the future: Photonic crystals which prohibit the propagation of light, or allow it only in certain directions at certain frequencies, or localize light in specific areas. This has vast implications for physicists, materials scientists, and electrical engineers and suggests such possible developments as an entirely optical computer.