Explaining Common Parameters Another frequently quoted parameter is the fill ratio. The fill ratio defines the ratio of the feature to the lattice cell in terms of area or volume for three dimensional crystals. Sometime a feature ratio of radius, or feature 'width' to the lattice spacing is used although this can always be turned into a fill ratio. The lattice spacing constant for a cubic lattice is the smallest distance that you must travel to get to a similar lattice point. Therefore we can choose to work with either feature ratios or fill ratios depending on which is more appropriate.
Lattices can range from square, cubic, hexagonal, tetragonal etc.
For those people that are familiar with the Bragg interference condition then a photonic crystal uses the very same. Consider a plane wave incident on a periodic structure such as the first row of cylinders in a two dimensional photonic crystal. As a normal wave is incident on the structure it may diffract and scatters but it will also split into forward and reverse waves. As the forward wave continues into the crystal is encounters more and more obstacles that cause more diffraction, scattering and forward and reverse waves. The periodic spacing of the cylinders may lead to the forward and reverse waves inside the crystal to destructively interfere with each other.