Healing Near-PT-Symmetric Structures to Restore Their Characteristic Singularities: Analysis and Examples

PT-symmetric structures, such as a pair of coupled waveguides with balanced loss/gain, exhibit a singularity of their eigenvalues around an exceptional point, hence a large apparent differential gain. In the case of fixed losses and variable gain, typical of plasmonic systems, a similar behavior emerges but the singularity is smoothened, especially in more confined structures. This reduces the differential gain around the singular point. Our analysis ascribes the origin of this behavior to a complex coupling between the waveguides once gain is present in an unsymmetrical fashion, even if guides feature the same modal gains in isolation. We demonstrate that adjunction of a real index variation to the variable waveguide heals the singularity nearly perfectly, as it restores real coupling. We illustrate the success of the approach with two geometries, planar or channel, and with different underlying physics, namely dielectric or plasmonic.