An operator-based approach to topological photonics

TL;DR

This paper introduces an operator-based theoretical framework to analyze topological properties of photonic structures directly from their Hamiltonian and position operators, enabling study of systems beyond traditional band theory limitations.

Contribution

It develops a novel approach to assess photonic topology without relying on band structures, revealing topological states in systems without complete band gaps and introducing symmetry-based invariants.

Findings

Topological states can exist in photonic crystals without complete band gaps.

The framework predicts boundary-localized chiral resonances in non-reciprocal photonic systems.

New invariants based on crystalline symmetries enable robust localization predictions.

Abstract

Recently, the study of topological structures in photonics has garnered significant interest, as these systems can realize robust, non-reciprocal chiral edge states and cavity-like confined states that have applications in both linear and non-linear devices. However, current band theoretic approaches to understanding topology in photonic systems yield fundamental limitations on the classes of structures that can be studied. Here, we develop a theoretical framework for assessing a photonic structure's topology directly from its effective Hamiltonian and position operators, as expressed in real space, and without the need to calculate the system's Bloch eigenstates or band structure. Using this framework, we show that non-trivial topology, and associated boundary-localized chiral resonances, can manifest in photonic crystals with broken time-reversal symmetry that lack a complete band gap, a result which may have implications for new topological laser designs. Finally, we use our operator-based framework to develop a novel class of invariants for topology stemming from a system's crystalline symmetries, which allows for the prediction of robust localized states for creating waveguides and cavities.