Toroidal dipolar bound state in the continuum and antiferromagnetic in asymmetric metasurface

Abstract

The toroidal dipole (TD) with weak coupling to the electromagnetic fields promises tremendous capability in sensing, light absorption and optical nonlinearities. Here, we analyze the near-field coupling effects of an asymmetric all-dielectric metasurface with an array of high-index Mie-resonant dielectric tetramer cluster. Two distinct optical toroidal dipolar bound states in the continuum are identified from the asymmetric metasurface. One occurs at the internal gap of the tetramer cluster array (intra-cluster toroidal modes), and the other arises from two neighboring clusters (inter-cluster toroidal modes). Via control of the asymmetry of the cluster, the TD could transform from bound states in the continuum into leaky resonances with high-Q factors. The optical antiferromagnetic response could also be identified from the tetramer cluster arrays. The toroidal and antiferromagnetic effects come from the trapped modes with symmetry breaking. These robust responses of the asymmetric metasurface remain stable as the asymmetry degree increases and the polarization of the incident light changes. The proposed metasurface with efficient light–matter interaction serves as a platform for controlling and exploiting optical toroidal and antiferromagnetic excitations.