A Numerical Mode Matching Method for Wave Scattering in a Layered Medium with a Stratified Inhomogeneity

Abstract

Numerical mode matching (NMM) methods are widely used for analyzing wave propagation and scattering in structures that are piecewise uniform along one spatial direction. For open structures that are unbounded in transverse directions (perpendicular to the uniform direction), the NMM methods use the perfectly matched layer (PML) technique to truncate the transverse variables. When incident waves are specified in homogeneous media surrounding the main structure, the total field is not always outgoing, and the NMM methods rely on reference solutions for each uniform segment. Existing NMM methods have difficulty handing grazing incident waves and special incident waves related to the onset of total internal reflection, and are not very efficient at computing reference solutions for nonplane incident waves. In this paper, a new NMM method is developed to overcome these limitations. A hybrid Dirichlet-Robin boundary condition is proposed to ensure that nonpropagating and nondecaying wave field components are not reflected by truncated PMLs. Exponential convergence of the PML solutions based on the hybrid Dirichlet--Robin boundary condition is established theoretically. A fast method is developed for computing reference solutions for cylindrical incident waves. The new NMM is implemented for two-dimensional structures and polarized electromagnetic waves. Numerical experiments are carried out to validate the new NMM method and to demonstrate its performance.