Photonic bilayer Chern insulator with corner states

TL;DR

This paper demonstrates a photonic bilayer Chern insulator where interlayer coupling induces a topological phase transition, leading to gapped edge states and the emergence of robust corner states with potential for defect-resistant photonic applications.

Contribution

First experimental realization of a bilayer gyromagnetic photonic crystal showing a transition from Chern insulator to higher-order topological phase with corner states.

Findings

Interlayer coupling causes a topological phase transition.

Corner states emerge within the bandgap.

Corner states show high resilience to defects.

Abstract

Photonic Chern insulators can be implemented in gyromagnetic photonic crystals with broken time-reversal (TR) symmetry. They exhibit gapless chiral edge states (CESs), enabling unidirectional propagation and demonstrating exceptional resilience to localization even in the presence of defects or disorders. However, when two Chern insulators with opposite Chern numbers are stacked together, this one-way nature can be nullified, causing the originally gapless CESs to become gapped. Recent theoretical works have proposed achieving such a topological phase transition in condensed matter systems using antiferromagnetic thin films such as MnBi2Te4 or by coupling two quantum spin/anomalous Hall insulators, but these approaches have yet to be realized experimentally. In a bilayer gyromagnetic photonic crystal arranged in an antiferromagnetic layer configuration, our experimental observations reveal that interlayer coupling initiates a transition from a Chern insulating phase to a higher-order topological phase. This transition results in the gapping of CESs and triggers the emergence of corner states within the bandgap. The corner mode energy within the gap can be attributed to CESs interaction, forming a Jackiw-Rebbi topological domain wall mode at the corner. These states exhibit heightened resilience against defects, setting them apart from their time-reversal symmetric counterparts.