Topological bands and triply-degenerate points in non-Hermitian hyperbolic metamaterials

TL;DR

This paper develops a non-Hermitian topological band theory for hyperbolic metamaterials, revealing triply-degenerate points and topological edge states, confirmed by numerical simulations, advancing understanding of topological photonic continua.

Contribution

It introduces a novel non-Hermitian Hamiltonian framework for continuous HMMs, uncovering new topological features and triply-degenerate points induced by chiral and gyromagnetic effects.

Findings

Identification of two types of triply-degenerate points with topological charges ±2 and 0.

Numerical confirmation of topological edge states via Maxwell's equations simulations.

Role of vacuum band in topological properties and bulk-edge correspondence.

Abstract

Hyperbolic metamaterials (HMMs), an unusual class of electromagnetic metamaterials, have found important applications in various fields due to their distinctive properties. A surprising feature of HMMs is that even continuous HMMs can possess topological edge modes. However, previous studies based on equal-frequency surface (analogy of Fermi surface) may not correctly capture the topology of entire bands. Here we develop a topological band description for continuous HMMs that can be described by a non-Hermitian Hamiltonian formulated from Maxwell's equations. We find two types of three dimensional non-Hermitian triply-degenerate points with complex linear dispersions and topological charges $\pm 2$ and 0 induced by chiral and gyromagnetic effects. Because of the photonic nature, the vacuum band plays an important role for topological edge states and bulk-edge correspondence in HMMs. The topological band results are numerically confirmed by direct simulation of Maxwell's equations. Our work presents a general non-Hermitian topological band treatment of continuous HMMs, paving the way for exploring interesting topological phases in photonic continua and device implementations of topological HMMs.