Disorder-induced scattering loss of line-defect waveguides in photonic crystal slabs

Detailed propagation loss spectrum measurements for line-defect waveguides in silicon photonic crystal slabs are presented, which show record low loss values $(5\mathrm{dB}∕\mathrm{cm})$ and complicated frequency dependence. We quantitatively analyze the origin of the loss spectrum shape using a photon Green function theory and obtain a very good agreement, thus providing an explanation of the complex physical mechanisms responsible for the observed propagation loss. In particular, we demonstrate the influence of out-plane, backward, intermode, and in-plane scattering processes on the observed loss spectra, induced by the structural disorder that occurs during fabrication, and highlight the importance of backward and intermode scattering in these waveguides.