Obstructed Atomic Limit Topological Protection in C4-Symmetric Photonic Crystals for Optical Communications

TL;DR

This paper explores how symmetry-enforced polarization mismatches in C4-symmetric photonic crystals can create interface modes that enable robust, high-transmission optical communication channels, despite structural imperfections.

Contribution

It demonstrates the practical use of obstructed atomic limit polarization mismatches to engineer robust photonic interface modes in C4-symmetric systems.

Findings

Interface modes are stabilized by Wannier center mismatches.

Nearly lossless transmission through sharp turns and defects.

Increasing crystal periods improves confinement and stability.

Abstract

Recent developments in photonic topological phases have revealed that protected edge modes can emerge not only from global topological invariants, but also from symmetry-enforced polarization mismatches between distinct bulk phases. In this work, we investigate the capabilities and limitations of a square-lattice ($C_4$-symmetric) photonic crystal composed of a single dielectric material that supports interface-localized modes at the boundary between regions characterized by distinct obstructed atomic limits (OALs). These modes are confined to a common band gap and exhibit high transmission, even in the presence of structural perturbations. Our analysis reveals that the interface modes are stabilized by a mismatch in the position of Wannier centers between the two adjoining crystals. We demonstrate nearly lossless transmission around sharp turns and through localized defects, though the robustness depends asymmetrically on the side of perturbation, reflecting the partial nature of the protection. We also show that increasing the number of photonic crystal periods surrounding the interface enhances both modal confinement and spectral stability. These findings establish polarization mismatch between OALs as a practical and fabrication-compatible mechanism for engineering robust photonic transport in \(C_4\)-symmetric systems.