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.
