In order to create an optical device with a photonic band gap extending in two
dimensions and with very uniform properties in any direction and for any polarisation
state, to within 1%, air holes are etched within a substrate of low refractive
index material such silicon oxynitride or silica glass. The ratio of air hole area
to the remainder of the substrate is low, being less than 35%. The air holes define
a quasicrystal structure, having twelve fold symmetry, being based on a square-triangle
system. In another development, an etched substrate with a regular crystal structure
or quasicrystal structure exhibits a non-linear refractive index. Two adjacent
areas in such a substrate have different lattice properties, or have defects in
the lattices, to create a unidirectional transmission path (diode action). A further
beam of light may be used to modulate the transmission path by reason of the non-linear
refractive index.
