Intrinsic triple degeneracy point bounded by nodal surfaces in chiral photonic crystal

TL;DR

This paper reveals an intrinsic triple degeneracy point at zero frequency in chiral photonic crystals, characterized by topological charges and protected by screw symmetry, with observable surface Fermi-arcs indicating non-trivial topology.

Contribution

It demonstrates the existence of an intrinsic triple degeneracy point with topological charge in chiral photonic crystals, and shows how screw symmetry reveals embedded topological features.

Findings

Intrinsic triple degeneracy point with topological charge

Nodal surface enclosing the Gamma point

Emergent Fermi-arcs connecting topological singularities

Abstract

In periodic systems, band degeneracies are usually protected and classified by spatial symmetries. However, the Gamma point at zero-frequency of a photonic system is an intrinsic degeneracy due to the polarization degree of freedom of electromagnetic waves. We show here that in chiral photonic crystals, such an intrinsic degeneracy node carries +(-)2 chiral topological charge and the topological characters is the same as a spin-1 Weyl point manifested as a triple degeneracy of two linear propagating bands intersecting a flat band representing the electrostatic solution. Such an intrinsic triple degeneracy point (TDP) at Gamma is usually buried in bulk band projections and the topological charge at photonic zero-frequency has never been observed. Here, by imposing space-group screw symmetry to the chiral photonic crystal, the Brillouin zone boundary is transformed into an oppositely charged nodal surface enclosing the Gamma point. The emergent Fermi-arcs on sample surface are then forced to connect the bulk band projections of these topological singularities, revealing the embedded non-trivial topology.