Theory of intrinsic propagation losses in topological edge states of planar photonic crystals

TL;DR

This paper develops a semi-analytic theory to analyze intrinsic propagation losses in topological photonic crystal waveguides, revealing significant losses in some structures and promising low-loss modes in others, aiding design of robust photonic devices.

Contribution

It provides the first detailed theoretical analysis of intrinsic losses in topological photonic crystal edge states, clarifying their loss mechanisms and guiding future waveguide design.

Findings

Armchair edge states exhibit >100 dB/cm intrinsic losses.

Valley Hall and inversion symmetry structures have low-loss modes.

Some topological structures offer bandwidth for lossless propagation.

Abstract

Using a semi-analytic guided-mode expansion technique, we present theory and analysis of intrinsic propagation losses for topological photonic crystal slab waveguide structures with modified honeycomb lattices of circular or triangular holes. Although conventional photonic crystal waveguide structures, such as the W1 waveguide, have been designed to have lossless propagation modes, they are prone to disorder-induced losses and backscattering. Topological structures have been proposed to help mitigate this effect as their photonic edge states may allow for topological protection. However, the intrinsic propagation losses of these structures are not well understood and the concept of the light line can become blurred. For four example topological edge state structures, photonic band diagrams, loss parameters, and electromagnetic fields of the guided modes are computed. Two of these structures, based on armchair edge states, are found to have significant intrinsic losses for modes inside the photonic band gap, more than 100 dB/cm, which is comparable to or larger than typical disorder-induced losses using slow-light modes in conventional photonic crystal waveguides, while the other two structures, using the valley Hall effect and inversion symmetry, are found to have a good bandwidth for exploiting lossless propagation modes below the light line.