Retardaion-induced exceptional point

TL;DR

This paper investigates how retardation effects induce exceptional points in optical dimers, revealing new non-Hermitian phenomena arising from the nonlinear eigenfrequency equations due to radiative coupling.

Contribution

It demonstrates that retardation causes exceptional points in symmetric and PT-symmetric dimers, highlighting a novel mechanism for non-Hermitian degeneracies in optical systems.

Findings

Exceptional points can occur without material contrast due to retardation.

Retardation leads to nonlinear eigenfrequency equations.

Transition between PT-symmetric and symmetric regimes is studied.

Abstract

Exceptional points in an optical dimer of spheres, which have the same size and operate in the spectral region of the dipolar resonance, are considered. By choosing different materials of these spheres, we can offset the radiative loss and create a gain-loss contrast to achieve a parity-time (PT)-symmetric dimer. In this case, an exceptional point corresponds to the point where the PT symmetry is broken. At the same time, if we consider a symmetric dimer, where both spheres are made of the same material (which may have a purely real dielectric constant), exceptional points occurring due to the radiative loss non-Hermiticity can also be observed. We study the transition between the two regimes and demonstrate that the exceptional point emerges due to the retardative nature of the coupling between the spheres, which makes the equation for eigenfrequencies nonlinear and allows it to have nontrivial solutions even when there is no contrast between the spheres.