Partial Topological Protection in C4 Lattices for Optical Communications

Abstract: In recent studies, analogs of the electronic Quantum Spin-Hall Effect have been explored within photonic crystals that incorporate spatial symmetries, especially those with $ C_{6v} $ symmetry, where $ \mathbb{Z}_2 $ topological invariants are enforced by crystalline symmetry. These photonic crystals possess bulk states with well-defined pseudospins and exhibit helical edge states, closely resembling their electronic counterparts. However, achieving $\mathbb{Z}_2$ topological protection in a square lattice photonic crystal remains great theoretical and experimental challange. In this work, we propose a single material photonic crystal structure based on a $ C_4 $ lattice that supports partially $ \mathbb{Z}_2 $-protected edge modes. We show that this structure can host photonic band-gap that hosts $ \mathbb{Z}_2 $-like modes, enabling perfect transmission in waveguide applications. Furthermore, we investigate the robustness of these modes against structural defects and directional turns, highlighting the distinctions between full $ \mathbb{Z}_2 $ topological protection and partial topological protection. Finally, we analyze the impact of the number of elementary cells surrounding the interface on the formation and stability of these protected modes.