Tailoring the excitation of two kinds of toroidal dipoles in all-dielectric metasurfaces

Abstract

In this paper, the simultaneous excitation of the toroidal dipoles within the unit cell and between neighboring unit cells is theoretically proposed and demonstrated in an all-dielectric metasurface composed of silicon bars. We referred to the toroidal dipole moment within the unit cell as the intra-toroidal dipole moment, and the toroidal dipole moment between the unit cells as the inter-toroidal dipole moment. We found that the contribution of the weak free-space coupling of the inter-toroidal dipole and intra-toroidal dipole can be dramatically increased by decreasing the distance between the adjacent silicon bars, so the electric and magnetic field is significantly confined both inside and between the silicon bars. Hence the radiation loss is greatly suppressed and a toroidal resonance with the high Q-factor of 63,000 is achieved. The proposed all-dielectric metasurface is low loss, simple structure and operating in the near infrared regime, which has potential applications in biochemical sensing, non-linear devices, slow light-based devices, and other optical devices.