Robust exceptional point of arbitrary order in coupled spinning cylinders

TL;DR

This paper demonstrates a method to achieve exceptional points of any order in coupled spinning cylinders on a waveguide, enabling robust non-Hermitian degeneracies with potential applications in optical isolation and topological photonics.

Contribution

The authors introduce a novel mechanism using non-reciprocal coupling of spinning cylinders to realize arbitrary-order exceptional points in a robust manner.

Findings

Achieved arbitrary-order EPs via non-reciprocal spinning cylinder coupling.

EPs are robust against spin-flipping perturbations.

Higher-order EPs enhance optical isolation, useful for nonreciprocal devices.

Abstract

Exceptional points (EPs), i.e., non-Hermitian degeneracies at which eigenvalues and eigenvectors coalesce, can be realized by tuning the gain/loss contrast of different modes in non-Hermitian systems or by engineering the asymmetric coupling of modes. Here we demonstrate a mechanism that can achieve EPs of arbitrary order by employing the non-reciprocal coupling of spinning cylinders sitting on a dielectric waveguide. The spinning motion breaks the time-reversal symmetry and removes the degeneracy of opposite chiral modes of the cylinders. Under the excitation of a linearly polarized plane wave, the chiral mode of one cylinder can unidirectionally couple to the same mode of the other cylinder via the spin-orbit interaction associated with the evanescent wave of the waveguide. The structure can give rise to arbitrary-order EPs that are robust against spin-flipping perturbations, in contrast to conventional systems relying on spin-selective excitations. In addition, we show that higher-order EPs in the proposed system are accompanied by enhanced optical isolation, which may find applications in designing novel optical isolators, nonreciprocal optical devices, and topological photonics.