Experimental Observation of Toroidal Dipole Modes in All-Dielectric Metasurfaces

The study of toroidal dipole modes has attracted a growing attention due to the specific properties of the toroidal electromagnetic response which differs from more familiar electric and magnetic dipole modes. Herein, toroidal dipole modes generated by metasurfaces composed of trimer clusters of high-index dielectric particles are observed. Both far-field transmission measurements and direct near-field mapping of the electromagnetic fields are performed in microwave experiments, and two distinct types of the toroidal dipole modes are observed in a single geometry of the metasurface design, where the toroidal modes are generated either inside of the three-particle clusters (the so-called intra-cluster toroidal modes) or between the neighboring particles in the clusters (inter-cluster toroidal modes). A transient response of the toroidal dipole modes excited by a pulse is studied in detail. Since the metasurface is composed of simple dielectric disks without the use of any metallic components, the proposed design can be feasibly scalable to both micro- and nanometer-size dielectric structures, and it can be employed in the flat-optics platform for realizing the beams shaping and efficient light-matter interaction for multiple hotspot energy localization, nonlinear frequency conversion, and highly efficient trapping of light.