Photonic axion insulator
Gui-Geng Liu, Subhaskar Mandal, Xiang Xi, Qiang Wang, Chiara Devescovi, Antonio Morales-Pérez, Ziyao Wang, Linyun Yang, Rimi Banerjee, Yang Long, Yan Meng, Peiheng Zhou, Zhen Gao, Y. D. Chong, Aitzol García‐Etxarri, Maia G. Vergniory, Baile Zhang
Abstract
Axions, hypothetical elementary particles that remain undetectable in nature, can arise as quasiparticles in three-dimensional crystals known as axion insulators. Previous implementations of axion insulators have largely been limited to two-dimensional systems, leaving their topological properties in three dimensions unexplored in experiment. Here, we realize an axion insulator in a three-dimensional photonic crystal and probe its topological properties. Demonstrated features include half-quantized Chern numbers on each surface that resembles a fractional Chern insulator, unidirectional chiral hinge states forming topological transport in three dimensions, and arithmetic operations between fractional and integer Chern numbers. Our work experimentally establishes the axion insulator as a three-dimensional topological phase of matter and enables chiral states to form complex, unidirectional three-dimensional networks through braiding.