The null field approach to electromagnetic scattering from composite objects: the case of concavo-convex constituents

Abstract
Time-harmonic electromagnetic scattering from a composite body consisting of a (dielectric or metallic) core plus one or several dielectric coatings was studied using the null-field approach. Previously developed null-field approaches to scattering from composite bodies do not apply when these coatings are of concavo-convex shapes. The authors examine this case and develop alternative null-field approaches to such geometries. While the scattering problem is usually solved by determination of the total transition matrix, referring to spherical waves, for the composite scatterer, the authors' approaches lead to different algebraic expressions for the transition matrix. Two main alternatives are studied. One of these makes use of Q-matrices for open surfaces while the other is based on a limit procedure applied to a previously developed formalism for layered scatterers. The numerical accuracy of the results is less than that obtained for homogeneous scatterers of similar exterior shape and electrical size. The convergence of the numerical implementation of the equations is studied in terms of several indicators such as dependence on the truncation order, the accuracy with which general constraints such as symmetry and unitarity are fulfilled, and the influence of different choices of expansion functions obtained from a moment-method solution.