Photonic Bilayer Chern Insulator with Corner States.

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 MnBi_{2}Te_{4} 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 band gap. The corner mode energy within this gap originates from CES interactions, forming a Jackiw-Rebbi-type topological domain wall mode at the corner, which is expected to remain within the bulk band gap without relying on local symmetries such as chiral or particle-hole symmetry. These states exhibit heightened resilience against defects, distinguishing them from their TR-symmetric counterparts.