Creating a gap without symmetry breaking in two-dimensional photonic crystals

Degeneracy can be broken and spectral gap (including absolute gap) can be created by adding a small fraction of metallic component at the center of each dielectric cylinder without breaking the space group symmetry in two-dimensional photonic crystals. This phenomenon can occur for both normal and accidental degeneracies and does not exist in electron systems. Physical explanations are given for both cases. This facilitates the opening of large absolute gaps. The method can be generalized to three-dimensional photonic crystals and opens a unique way to engineer photonic band gaps.