Photonic-crystal waveguides with disorder: Measurement of a band-edge tail in the density of states

Summary form only given. Photonic-crystal waveguides are excellent model systems to investigate band-gap related phenomena in condensed matter physics. An example is the effect of disorder on the band edge. Even for state-of-the-art systems, the smallest effects of disorder are enhanced in the so-called slow-light regime, eventually leading to Anderson localization and, hence, breakdown of the transport of light. We employ phase-sensitive near-field optical microscopy to measure spatial mode profiles at the band edge of such photonic-crystal waveguides. Both localized and extended modes are found. By spatial Fourier transforms, high-resolution band structures are obtained. From these the optical density of states (DOS) is retrieved. 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 found to be absent. The Anderson-localized states form a ‘Lifshitz tail’ in the DOS. Our results show the potential of studying band edge phenomena crucial for condensed-matter physics with artificial photonic structures.