Measuring the spatial extent of individual localized photonic states

We measure the spatial extent of individual photonic states that are localized by residual disorder in a slow-light photonic crystal waveguide. The size of the states is measured by locally perturbing them individually through an electromagnetic interaction with a near-field probe. We identify localized states that are not observed in transmission and show that they are shorter than the waveguide length. We also obtain near-field measurements of the participation ratio, from which the size of the states is also derived, in quantitative agreement with the size measured with the perturbation method.