Non-Hermitian Second-Order Topological Phases and Bipolar Skin Effect in Photonic Kagome Crystals

TL;DR

This paper explores non-Hermitian second-order topological phases in photonic kagome crystals, revealing how non-Hermiticity induces a bipolar skin effect and breaks traditional bulk-boundary correspondence, advancing topological photonics.

Contribution

It introduces a framework for non-Hermitian higher-order topological phases in photonic systems, linking topology with the skin effect and demonstrating new localization phenomena.

Findings

Non-Hermiticity lifts degeneracy of topological corner modes.

Bulk states accumulate at corners, causing bipolar skin effect.

Breakdown of conventional bulk-boundary correspondence.

Abstract

Non-Hermitian photonics provides a fertile platform for exploring phenomena with no Hermitian counterparts, including the non-Hermitian skin effect and exceptional points, with direct relevance for integrated photonic technologies. In this work, we investigate the properties of non-Hermitian second-order topological phases by constructing a photonic kagome crystal with balanced gain and loss, and reveal the interplay between higher-order topology and the non-Hermitian skin effect. We demonstrate that non-Hermiticity not only lifts the degeneracy of the topological corner modes but also drives bulk states to accumulate at corners, giving rise to bipolar non-Hermitian skin effect. By defining the point-gap topology, we uncover the fundamental topological origin of the non-Hermitian skin effect. More interestingly, the non-Hermitian skin effect induces a fundamental breakdown of the conventional bulk-boundary correspondence based on the Bloch band theory. Our findings establish a general framework for non-Hermitian higher-order photonic systems and open avenues toward tailorable topological photonic devices exploiting non-Hermitian enhanced localization.