Performance changes of a grated waveguide at resonance wavelengths next to its band-edges due to modified edge sections

An efficient numerical scheme developed on the basis of Green’s function method is applied to the investigation of structural effects on the performance of planar grated waveguide at the first resonance wavelengths next to the band-edges. Restricting ourselves to the transverse-electric waves, this study is focused on the effects induced by variations of the grating cell number and the depths of its four outer grooves on both sides. The different patterns of groove depth gradation or apodization considered in this study are all characterized by decreasing depth toward the ends while retaining the longitudinal grating symmetry. The effects of the modifications are expressed in terms of changes in the modal transmittance, reflectance, and out-of-plane scattering loss as well as the group velocity and resonant field enhancement. The most favorable result characterized by 15% transmittance enhancement and 85% loss reduction is achieved for the case with the most gradual changes in the groove depth. It is further shown that, for the investigated range of parameters, both the group velocity and field enhancement can best be improved by increasing the length of the uniform grating, without introducing any modification.