EXACT-EIGENMODE MODEL FOR INDEX-MODULATED APERTURES

We consider diffraction of an arbitrary two-dimensional electromagneti c field by a lamellar dielectric structure of arbitrary form, which fi lls a slit aperture in a conducting screen of finite thickness. The pr oblem is treated rigorously, by expanding the field inside the index-m odulated aperture in the form of a series of exact eigenfunctions. Thi s discrete set of eigenfunctions is obtained by means of the theory of stratified media, combined with the appropriate boundary conditions a t the perfectly conducting edges of the aperture. Then the eigenmode e xpansion is matched to the angular spectrum representations of the inc ident, forward-diffracted, and backward-diffracted fields to determine the total field everywhere in space. Numerical implementation and con vergence properties of the method are discussed. Some illustrations of diffraction by nonperiodic structures are given. (C) 1996 Optical Soc iety of America.