Two mechanisms, three stages of the localization of light in a disordered dielectric structure with photonic band gaps

We study the localization of light in the disordered dielectric structures derived from periodic systems with photonic band gaps. It is directly shown by field pattern that with moderate disorder there exist strongly localized states in the original gap. The localization-delocalization-localization trilogy discovered in this paper embodies the concept ``interplay between the order and disorder'' that has been previously suggested, and it is pointed out that the complete understanding of this complex localization behavior should take into account two localization mechanisms with different ranges of coherence. The difference between the two localization mechanisms is demonstrated by the different localization behaviors of the TM and TE modes. The C number is introduced to show the localization centers are frequency dependent. The frequency distribution of localization centers is found to vary with the degree of disorder.