Photonic Crystals
| Structured optical materials where the refractive index varies periodically in two or three directions are termed Photonic Crystals. When the scale of the periodicity is comparable to the wavelength, Bragg reflection occurs, but only over wavelength ranges that depend on the propagation direction in the crystal and the periodic refractive index modulation. These "band gaps" are analogous to the electronic conduction band gaps found in semiconducting materials. Some photonic crystals have complete band gaps. In this case - light cannot propagate in the photonic crystal in any direction and all light incident on the crystal will be reflected, regardless of direction. | |
  Electron micrographs of a butterfly wing show a regular crystalline structure that produces colour-variable reflection |
Photonic crystals occur in nature.The coloured reflection from butterfly wings, for example, comes from wavelength-selective reflection of light from two and three dimensional microstructured features on the surface of the wing. Synthetic photonic crystals with tailored optical characteristics can also be fabricated, in silicon for example using semiconductor fabrication techniques. |
| Photonic crystals will be a key enabling technology for the development of microphotonic devices. Consider, for example, a photonic crystal with a row of periods taken out. This acts as a waveguide for wavelengths within a band gap, since this light cannot "leak away" through the crystal. In principle,then, waveguides in photonic crystals can guide light through 90° bends with micrometre-scale radii of curvature. The enormous dispersion at frequencies close to the edges of the photonic band gap is strongly dependent on propagation direction and wavelength, so small changes in incident direction or wavelength may lead to large changes in propagation direction inside the photonic crystal. This "superprism" is the basis of the CUDOS logo. |  Artist's inmpression of a three dimensional photonic crystal, so called because the structure is periodic ("crystalline") in all three dimensions |
