Observation of photonic antichiral edge states

TL;DR

This paper reports the experimental discovery of antichiral edge states in a gyromagnetic photonic crystal, revealing novel propagation behaviors that challenge traditional topological edge state theories.

Contribution

The first experimental observation of antichiral edge states in a photonic system, demonstrating their unique properties and expanding topological photonics understanding.

Findings

Antichiral edge states propagate in the same direction along both edges.

Unique dispersion and robust propagation of antichiral states observed.

Scattering into backward bulk states at certain terminations was detected.

Abstract

Chiral edge states are a hallmark feature of two-dimensional topological materials. Such states must propagate along the edges of the bulk either clockwise or counterclockwise, and thus produce oppositely propagating edge states along the two parallel edges of a strip sample. However, recent theories have predicted a counterintuitive picture, where the two edge states at the two parallel strip edges can propagate in the same direction; these anomalous topological edge states are named as antichiral edge states. Here we report the experimental observation of antichiral edge states in a gyromagnetic photonic crystal. The crystal consists of gyromagnetic cylinders in a honeycomb lattice, with the two triangular sublattices magnetically biased in opposite directions. With microwave measurement, unique properties of antichiral edge states have been observed directly, which include the titled dispersion, the chiral-like robust propagation in samples with certain shapes, and the scattering into backward bulk states at certain terminations. These results extend and supplement the current understanding of chiral edge states.