Higher-order topological states in two-dimensional Stampfli-Triangle photonic crystals

In this Letter, the higher-order topological state (HOTS) and its mechanism in two-dimensional Stampfli-Triangle (2D S-T) photonic crystals (PhCs) is explored. The topological corner states (TCSs) in 2D S-T PhCs are based on two physical mechanisms: one is caused by the photonic quantum spin Hall effect (PQSHE), and the other is caused by the topological interface state. While the former leads to the spin-direction locked effect which can change the distribution of the TCSs, the latter is conducive to the emergence of multiband TCSs in the same structure due to the characteristics of plentiful photonic bandgap (PBG) and broadband in 2D S-T PhCs. These findings allow new, to the best of our knowledge, insight into the HOTS, and are significant to the future design of photonic microcavities, high-quality factor lasers, and other related integrated multiband photonic devices.