How does slow light propagate in a real photonic-crystal waveguides?

We report ensemble-average transport characteristics obtained for a series of photonic-crystal waveguides that are supposedly identical and that only differ because of statistical structural fabrication-induced imperfections. In particular, we evidence that, in addition to a smearing of the local density of states, the probability density function of the transmission rapidly broadens in the slow light regime even for group indices as small as n(g)approximate to 20 and for practical situations offering tolerable -3dB losses. This brings a severe constraint on the effective use of slow light for on-chip optical information processing. The experimental results are quantitatively supported by theoretical results obtained with a coupled-Bloch-mode approach that takes into account multiple scattering and localization effects.