Universality of mode symmetries in creating photonic Dirac cones

The creation of photonic Dirac cones by accidental degeneracy in the Brillouin-zone center was recently reported for both metamaterials with localized electromagnetic resonant states and dielectric photonic crystals without well-defined resonance. Based on the anticipation that there should be a common physical origin in this phenomenon, we systematically examined the relation between mode symmetries and shapes of dispersion curves for both systems. The result strongly suggests the presence of universality of mode symmetries that enable the creation of photonic Dirac cones irrespective of the details of the sample structure.