Critical States Embedded in the Continuum

TL;DR

This paper introduces a new class of critical states in optical waveguides with non-Hermitian defects, which are embedded in the continuum and exhibit a phase transition influenced by the defect's imaginary refractive index.

Contribution

The work uncovers critical states embedded in the continuum in non-Hermitian optical systems and describes their phase transition and mode segregation phenomena.

Findings

Critical states are embedded in the continuum with real propagation constants.

A phase transition driven by the defect's imaginary refractive index occurs.

Localized states can be turned into bound states in the continuum with background gain or loss.

Abstract

We introduce a class of critical states which are embedded in the continuum (CSC) of one-dimensional optical waveguide array with one non-Hermitian defect. These states are at the verge of being fractal and have real propagation constant. They emerge at a phase transition which is driven by the imaginary refractive index of the defect waveguide and it is accompanied by a mode segregation which reveals analogies with the Dicke super -radiance. Below this point the states are extended while above they evolve to exponentially localized modes. An addition of a background gain or loss can turn these localized states to bound states in the continuum.