Photonic non-Hermitian skin effect and non-Bloch bulk-boundary correspondence

In the area of non-Hermitian physics, there has been increasing research interest in photonics. Recently, this interest has expanded to topological systems in which symmetry and topology intertwine with non-Hermiticity, giving rise to many intriguing physical effects. One of the major tasks in exploring topological systems is unveiling the bulk-boundary correspondence in the presence of non-Hermiticity. Several proposals have been put forward in this vein, including non-Bloch bulk-boundary correspondence and the non-Hermitian skin effect. However, its practical realization has remained elusive thus far. In this paper, we demonstrate a feasible design of a one-dimensional non-Hermitian Su-Schrieffer-Heeger model based on photonic coupled resonant optical waveguides (CROWs). We show that non-Hermitian asymmetric coupling can be realized by the judicious design of optical gain and loss elements into unidirectional coupling link rings. The phase transition points of a technically achievable CROW open chain are different from those of the periodic boundary, thus revealing the non-Bloch bulk-boundary correspondence. Moreover, the field distribution is found to be exponentially localized at the ends of an open-boundary chain, which demonstrates the non-Hermitian skin effect. Our results underpin the fundamental importance as well as potential applications in various optical devices such as optical couplers, beam splitters, lasers, optical trapping, etc.