Coupling characteristics of localized photons in two-dimensional photonic crystals

We show that the coupling behavior between defect modes in photonic crystals can be quite different from that between electrons confined in atoms. The parity of the defect modes split due to the coupling is not conserved as the interdefect distance changes. This parity nonconservation is attributed to the oscillatory nature of the evanescent waves of localized photons. The rapid decay of the field of localized photons in some coupling directions makes the coupling between localized photons for those directions very weak even if the distance between the defects is very close. These characteristics of the coupling between the localized photons will be useful in the implementations of directional optical filters and of narrow band waveguides.