Momentum space toroidal moment in a photonic metamaterial
Biao Yang, Yan‐Gang Bi, Rui-Xing Zhang, Ruo-Yang Zhang, Oubo You, Zhihong Zhu, Jing Feng, Hong‐Bo Sun, C. T. Chan, Chao‐Xing Liu, Shuang Zhang
Abstract
Berry curvature, the counterpart of the magnetic field in the momentum space, plays a vital role in the transport of electrons in condensed matter physics. It also lays the foundation for the emerging field of topological physics. In the three-dimensional systems, much attention has been paid to Weyl points, which serve as sources and drains of Berry curvature. Here, we demonstrate a toroidal moment of Berry curvature with flux approaching to π in judiciously engineered metamaterials. The Berry curvature exhibits a vortex-like configuration without any source and drain in the momentum space. Experimentally, the presence of Berry curvature toroid is confirmed by the observation of conical-frustum shaped domain-wall states at the interfaces formed by two metamaterials with opposite toroidal moments.