Anomalous Propagation Loss in Photonic Crystal Waveguides

Propagation loss can occur in photonic crystal waveguides without complete optical confinement. We employ a highly efficient transfer-matrix method which allows for accurate and reliable extraction of the propagation loss even at an extremely low level. The results for a two-dimensional photonic crystal waveguide shows that the loss exponentially decays via the waveguide wall thickness. An anomalous phenomenon is found where the loss for guided modes near the upper band gap edge can be several orders of magnitude smaller than that for modes in the middle of the band gap. This anomaly can be well explained by the localization degree of guided modes at different frequency domains.

Figure 1

Schematic configuration of light propagation in a 2D PC waveguide solved by the TMM approach. Two perfect absorption layers (PALs) are placed at the boundaries of the supercell to account for wave leakage.

Figure 2

KKR calculation results for the field pattern along the waveguide axis excited by an external monochromatic point source at different frequencies and different waveguide wall thicknesses.

Figure 3

(a) TMM calculation results of the dispersion for waveguides with different wall thicknesses. For the sake of easier viewing, the curves are offset alternately. (b),(c) Overall pictures for the propagation loss as a function of the excitation frequency and the waveguide wall thickness calculated by both the TMM and KKR approaches.

Figure 4

Calculated field profile at three lines perpendicular to the waveguide axis at different frequencies. The line $z=0$ passes through the center of cylinders.