Finite element and perturbative study of buffered leaky planar waveguides

The effects of the presence of a high-index medium in the proximity of planar waveguiding structures that makes up buffered leaky waveguides, were studied using a finite element method (FEM) leaky mode solver and a perturbation method. Various phenomena observed in the FEM results were interpreted through the approximate analytical expressions derived using the perturbation method. The effect of the buffer layer thickness, the high-index medium refractive index, and the quasi-confinement of the modal field were investigated. The results show that the perturbation due to the high-index medium can lead to either an increase or a decrease of the real part of the effective index, and that the leakage loss of a TE-polarized mode is not always lower than TM-polarized mode of the same order. It was found that if the refractive index of the high-index medium goes to infinity, a leaky-wave structure evolves into a guided-wave structure.