We measure localized and extended mode profiles at the band edge of slow-light photonic-crystal waveguides using phase-sensitive near-field microscopy. High-resolution band structures are obtained and interpreted, allowing the retrieval of the optical density of states (DOS). This constitutes a first observation of the DOS of a periodic system with weak disorder. The Van Hove singularity in the DOS expected at the band edge of an ideal 1D periodic structure is removed by the disorder. The Anderson-localized states form a "tail" in the density of states, as predicted by Lifshitz for solid-state systems.

Phys. Rev. B

Huisman, S. R., Ctistis, G., Stobbe, S., Mosk, A., Herek, J., Lagendijk, A., … Pinkse, P. W. H. (2012). Measurement of a band-edge tail in the density of states of a photonic-crystal waveguide. Phys. Rev. B, 86(Article number: 155154), 1–5. doi:10.1103/PhysRevB.86.155154