Measurement of a band-edge tail in the density of states of a photonic-crystal waveguide

We investigate light transport near the band edge of slow-light photonic-crystal waveguides using phase-sensitive near-field microscopy. We obtain and interpret high-resolution band structures, allowing the retrieval of the optical density of states for a one-dimensional periodic system with weak intrinsic disorder. Because of this disorder, the band edge is smeared out and the van Hove singularity is removed. The density of states shows a decaying tail in the band gap corresponding to Anderson-localized modes, as predicted by Lifshitz for solid-state systems.