Introduction to photonic crystals
Photonic crystals are created when a strong periodic modulation of refractive index in a material causes photons to be localised in discrete energy states, giving rise to a photonic band structure.
This band structure is analogous to the electronic band structure of semiconductor materials, but is created using man-made periodic structures, rather than natural crystal structures. We are therefore free to use patterns that do not occur in nature and fabricate these in well understood materials such as silicon.
Photonic crystals are not crystals in the sense that sapphire or diamonds are crystals. Photonic crystals can be made of many materials into which a periodic structure or strong refractive index modulation is fabricated.
One feature of photonic crystals is the ability to form a photonic band gap (PBG) region. This represents a range of energies where no permitted states are allowed and the light of a wavelength residing within is forbidden to propagate.
The periodicity of a photonic crystal can be 1, 2 or 3 dimensional. Two-dimensional photonic crystals can be fabricated in two different forms:
• Photonic crystal fibres which have air-holes running along their length
• Planar photonic crystals, of the type designed and manufactured by Mesophotonics, that contain arrays of holes each a couple of hundred nanometers deep
Illustrative planar photonic crystal structure and associated SEM micrographs of fabricated structures.
Simplified band gap structure.