Anapole States and Toroidal Resonances Realized in Simple Gold Nanoplate-on-Mirror Structures

Optical anapoles are often realized with high-refractive-index dielectric nanoparticles or metallic metamaterials with complex geometries. In this work, anapole states and plasmon-induced toroidal resonances are observed on a simple plasmonic configuration, which is made of individual Au nanoplates supported on Au films. The nanoplate and the film are separated by a precisely sized dielectric spacer layer. Both the fundamental and higher-order anapole states are experimentally observed in the far-field scattering measurements and confirmed by electrodynamic simulations. The geometrical parameters of the nanoplate-on-mirror cavity are found to play sensitive and significant roles in altering the anapole excitation, which allows the resonant dip to be tailored from the visible to near-infrared region. In addition, magnetic plasmon resonance is also realized in the nanoplate-on-mirror system when the thickness of the spacer layer is increased. These results provide new insights for engineering simple plasmonic nanostructures with nontrivial yet controllable responses and developing anapole-based plasmonic applications.