Designing Topological Defect Lines Protected by Gauge-Dependent Symmetry Indicators

Translating the concept of topological phases to photonics enables a vast number of device designs with intriguing properties. Conventional topological invariants, however, often fail to explain emerging models that exploit crystal symmetries. The recent development of generalized, computationally efficient symmetry indicators allows for a broader understanding of topological phases, and is readily applied to electromagnetic systems. This paper shows how this fresh approach leads to very practical photonic and microwave designs, especially for robust wave guiding, and illustrates the important role that real-space information plays in determining the topology of a photonic crystal system.