Axial Toroidal Dipole Modes in All- Dielectric Trimer Metasurfaces

We demonstrate how to excite a resonant state resembling an axial toroidal dipole in metasurfaces made of particle clusters composed of three dielectric disks (trimers). The toroidal dipole response appears due to a symmetry breaking in the cluster geometry, via an energy release from an electromagnetic trapped (dark) mode. We consider several designs of metasurfaces where an asymmetry is introduced into the cluster by either employing disks of different radii or identical disks with holes. We discuss in detail the effect of different asymmetries in metasurface's geometry on characteristics of the axial toroidal dipole response.