Fragility of Unidirectional Transport in Weakly Disordered Photonic Chern Insulators

TL;DR

This paper uncovers how weak disorder in photonic Chern insulators can disrupt unidirectional edge transport by forming impurity bands that couple with edge states, challenging assumptions about their robustness.

Contribution

It reveals a new mechanism where weak disorder creates impurity bands that break topological protection, providing deeper understanding of disorder effects in photonic topological insulators.

Findings

Weak disorder can form impurity bands inside the topological gap.

Coupling between defect states and edge states enables breakdown of unidirectional transport.

Reciprocal necklace states formed by defect coupling disrupt topological robustness.

Abstract

Photonic Chern insulators enable unidirectional light transport protected by nontrivial band topology -- essential for robust photonic integrated circuits and error-free communication. However, disorder from impurities or defects inevitably exists in practical applications, yet how weak disorder affects topological chiral edge states remains insufficiently understood. Here, we reveal a previously unrecognized mechanism by which weak disorder can disrupt robust propagation of chiral edge states in photonic Chern insulators, despite the preservation of global topological invariants. By randomly replacing a small number of magnetized rods with nonmagnetized impurities in a magnetic photonic crystal, we find that when the excitation frequency approaches the single impurity defect state frequency, weak coupling between spatially extended defect states forms a topologically trivial impurity band inside the topological gap. This enables coexistence and coupling of defect states and chiral edge states. The reciprocal "necklace state" transport channels formed by coupled defect states break the expected unidirectional propagation in topological Chern insulators with weak disorder. Our work reveals that topological chiral edge state and disorder interactions are more intricate than previously understood and provides new insights into stability and control of topological transport in realistic applications.