Fano interference governs wave transport in disordered systems

Light localization in disordered systems and Bragg scattering in regular periodic structures are considered traditionally as two entirely opposite phenomena: disorder leads to degradation of coherent Bragg scattering whereas Anderson localization is suppressed by periodicity. Here we reveal a non-trivial link between these two phenomena, through the Fano interference between Bragg scattering and disorder-induced scattering, that triggers both localization and delocalization in random systems. We find unexpected transmission enhancement and spectrum inversion when the Bragg stop-bands are transformed into the Bragg pass-bands solely owing to disorder. Fano resonances are always associated with coherent scattering in regular systems, but our discovery of disorder-induced Fano resonances may provide novel insights into many features of the transport phenomena of photons, phonons, and electrons. Owning to ergodicity, the Fano resonance is a fingerprint feature for any realization of the structure with a certain degree of disorder.