Single-mode versus multi-mode 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 w0=sqrt(3)a (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=1.5w0. 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.