Refractive Index Sensing Using Symmetry-Engineered Higher-Order Topological Dipole- and Hexapole-Corner Modes

Topological photonic microcavities have been widely applied in the design of photonic devices due to their unique stability against structural defects. Because higher-order topological corner states can confine photons in a lower-dimensional space (0D corners), they offer significantly higher quality factors (Q-factors). Typically, higher-order topological corner states exist in square and Kagome lattices, where the electric field manifests as a monopole at the vertices of the structure. Here, by engineering the global structural symmetry, we construct topological corner states in the form of dipoles and hexapoles in square and Kagome lattices, respectively. Taking the response to ambient refractive index variations as an example, we demonstrate that these symmetry-engineered corner states can provide more sensitive detection, higher Q-factors, and stronger immunity to structural defects. Therefore, this work further expands the tuning mechanisms of higher-order topological photonics and highlights its application potential in highly stable and ultra-sensitive sensing.