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Introduction to Photonic Crystals

What are Photonic Crystals?

  • Photonic crystals are created when a periodic modulation of refractive index in a material causes photons to be localised in discrete energy states giving rise to a photonic band structure.

Simplified band gap 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 no occur in nature and fabricate these in user friendly materials such as silicon.
  • Photonic crystals are therefore not crystals in the sense that sapphire or diamonds are crystals. Rather photonic crystals can be made of any material into which a periodic structure is fabricated.
  • The energy range with no permitted states is known as a photonic band gap (PBG). And represents a range of wavelengths where light cannot propagate in the photonic crystal.
  • The periodicity of a photonic crystal can be in 1, 2 or 3 dimensions.
    • Two-dimensional photonic crystals can be fabricated in two different forms
      • Photonic crystal fibres which have 10’s of air-holes running along their length
      • Planar photonic crystals of the type manufactured by Mesophotonics that contain arrays of 100’s of air-holes each a couple of hundred nanometres deep

Illustrative planar photonic crystal structure

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