Realization of Fourth Order Exceptional Points of Degeneracy in Uniform Coupled-Waveguides

Abstract

We present the general conditions to realize fourth order exceptional points of degeneracy (EPD) in two uniform (i.e., invariant along z) coupled transmission lines (CTLs). The CTLs are modeled as distributed generic components where each line (when uncoupled) may support forward propagation, backward propagation or evanescent modes. We show the novel and important fact that a fourth order EPD may be supported in uniform CTLs when there is coupling between a forward propagating mode and an evancent mode. We also provide a microstrip possible implementation of a uniform CTL exhibiting fourth order EPD using a very subwavelength unit cell length.