Interface Modes in Honeycomb Topological Photonic Structures with Broken Reflection Symmetry

TL;DR

This paper develops a mathematical framework to analyze Dirac points and interface modes in honeycomb topological photonic structures with broken reflection symmetry, revealing topological phase transitions and interface states.

Contribution

The work introduces a boundary integral equation approach to prove the existence of interface modes in perturbed honeycomb photonic structures with broken reflection symmetry.

Findings

Dirac points are shown to exist at Brillouin zone vertices in symmetric honeycomb lattices.

Breaking reflection symmetry opens band gaps and induces topological phase differences.

Interface modes are proven to exist at the boundary of perturbed structures using boundary integral analysis.

Abstract

In this work, we present a mathematical theory for Dirac points and interface modes in honeycomb topological photonic structures consisting of impenetrable obstacles. Starting from a honeycomb lattice of obstacles attaining $120^\circ$-rotation symmetry and horizontal reflection symmetry, we apply the boundary integral equation method to show the existence of Dirac points for the first two bands at the vertices of the Brillouin zone. We then study interface modes in a joint honeycomb photonic structure, which consists of two periodic lattices obtained by perturbing the honeycomb one with Dirac points differently. The perturbations break the reflection symmetry of the system, as a result, they annihilate the Dirac points and generate two structures with different topological phases, which mimics the quantum valley Hall effect in topological insulators. We investigate the interface modes that decay exponentially away from the interface of the joint structure in several configurations with different interface geometries, including the zigzag interface, the armchair interface, and the rational interfaces. Using the layer potential technique and asymptotic analysis, we first characterize the band-gap opening for the two perturbed periodic structures and derive the asymptotic expansions of the Bloch modes near the band gap surfaces. By formulating the eigenvalue problem for each joint honeycomb structure using boundary integral equations over the interface and analyzing the characteristic values of the associated boundary integral operators, we prove the existence of interface modes when the perturbation is small.