Single-mode versus multimode behavior in silicon photonic crystal waveguides measured by attenuated total reflectance

Linear photonic crystal waveguides with different channel widths realized in silicon membranes are investigated by means of attenuated-total-reflectance (ATR) measurements. The dispersion of line-defect modes with both parities with respect to a vertical plane bisecting the waveguide channel is determined, thereby allowing one to distinguish between multimode and single-mode behavior. The presence of a single-mode frequency window in the guided-mode region below the light line is established not only for standard W1 waveguides with channel width w(0)=root 3a (i.e., a missing row of holes in the triangular lattice with lattice constant a), but also for W1.5 waveguides where the channel width is increased to w=1.5w(0). The results agree with theoretical predictions and might be important for the realization of linear photonic crystal waveguides with single-mode behavior and ultralow propagation losses.