Degeneracy and Split of Defect States in Photonic Crystals

One-dimensional photonic crystals with two or more structural defects are studied. We observed an interesting characteristic of transmission band structure of photonic crystals with defects using the transmission-matrix-method simulation. The transmission states in the wide photonic band gap caused by defects reveal degeneracy and split in certain conditions. Every split state is contributed by coupling of all defects in a photonic crystal. Using the tight-binding method, we obtain an approximate analytic expression for the split frequency of photonic crystals with two structural defects.