Abstract
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.
© 2012 IEEE
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