Tunable topological edge state in mirrored photonic crystal interface

We have developed a mirrored photonic crystal interface (MPCI) by applying a mirror operator to expanded honeycomb photonic crystals (EHPCs) exhibiting nontrivial band topology. This configuration supports a pair of interface states characterized by spin-momentum locking. Notably, by adjusting the geometric parameters of the EHPCs without altering their topological properties, we observe a process where these interface states close and reopen, indicating a topological phase transition. Further analysis reveals that the MPCIs function as interfaces between EHPCs with <i>C</i>₆- and <i>C</i>₃-symmetric configurations, representing nontrivial and trivial topological phases, respectively, separated by air gaps. Our findings highlight the tunable nature of topological edge states formed by a single type of photonic crystal, which could have potential applications in robust transport devices within photonic integrated circuits.