Defect states and commensurability in dual-period Al_xGa_1-xAs photonic crystal waveguides

Al_xGa_1-xAs waveguides with one-dimensional dual-period patterning, consisting of a short period lattice of semiconductor stripes with periodically placed defects, are investigated by surface coupling reflectivity. Defect states, characterized by flat dispersions and enhanced optical density in the defect region, are observed within an energy gap arising from the short period modulation of the structure. When the defect width is incommensurate with the short period, strong additional features due to band folding are observed. The experimental spectra and dispersions are found to be in very good agreement with the predictions of scattering-matrix calculations.