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Ray Methods for Evanescent Fields

This paper deals with a review of recent developments in the treatment of high frequency evanescent fields. Evanescent fields have conventionally been associated with guided modes. For dielectric waveguides, for example, the evanescent fields in a trapped (discrete) mode exist exterior to the guiding structure, and they are specified over the entire exterior cross section. This global character inherent in a modal description of evanescent fields is unnecessarily restrictive. By a recent generalization of ray-optical procedures far non-evanescent fields, asymptotic high frequency methods have been developed that permit the local tracking of evanescent fields. These fields are described to a lowest order of approximation by u(r¿) - A(r¿) exp[ikS(r¿)], where the phase S and the amplitude A are complex, and the wavenumber k is assumed to be large. Accommodated by this generalization are Gaussian beams, leaky waves, surface waves and waves on the dark side of caustics.