Ideal Unconventional Weyl Point in a Chiral Photonic Metamaterial

TL;DR

This paper reports the experimental realization of a symmetry-protected charge-2 Weyl point in a three-dimensional chiral photonic metamaterial, demonstrating unique surface states with broad bandwidth and robustness.

Contribution

It presents the first clean demonstration of an ideal unconventional Weyl point in photonics, with direct observation of associated surface states in a chiral microwave metamaterial.

Findings

Observation of a charge-2 Weyl point in photonics.

Surface states span a 22.7% frequency bandwidth.

Surface states show topological self-collimation and robustness.

Abstract

Unconventional Weyl points (WPs), carrying topological charge 2 or higher, possess interesting properties different from ordinary charge-1 WPs, including multiple Fermi arcs that stretch over a large portion of the Brillouin zone. Thus far, such WPs have been observed in chiral materials and acoustic metamaterials, but there has been no clean demonstration in photonics in which the unconventional photonic WPs are separated from trivial bands. We experimentally realize an ideal symmetry-protected photonic charge-2 WP in a three-dimensional topological chiral microwave metamaterial. We use field mapping to directly observe the projected bulk dispersion, as well as the two long surface arcs that form a noncontractible loop wrapping around the surface Brillouin zone. The surface states span a record-wide frequency window of around 22.7% relative bandwidth. We demonstrate that the surface states exhibit a novel topological self-collimation property and are robust against disorder. This work provides an ideal photonic platform for exploring fundamental physics and applications of unconventional WPs.