Strong Field Enhancement Based on Manipuation of Anapole State in Dielectric Metasurface

Resonant metamaterials and metasurfaces cause by light-matter interaction in sub wavelength structures have been a hot research topic recently due to its potential in progressing the conventional devices. Anapole, a new hybrid mode is formed by overlapping toroidal dipole (TD) and electric dipole (ED) mode. Since the far field radiations of TD and ED are antiphase, destructive interference between them will turn this hybrid mode into a dark mode with strong near field. In addition, the electric field can be further enhanced by incorporating the slot effect, which happens to fulfill the continuity of electric displacement field at interfaces of low-refractive-index and high-refractive-index material. In this work, we propose a dielectric metasurface which can support anapole mode and introduce slot effect to manipulate field enhancement. The structure is composed of a concentric circle and ring, with a middle bar connecting them. We find the middle bar and gap are critical and give rise to the tunability. From the multipole decomposition, a combination of TD and ED with same amplitude can be corresponded to the anapole mode. It shows that the middle bar creates a path for displacement current passing through it, supporting the nonradiative mode with high quality factor. The strong near field enhancement and high Q demonstrated is believed to be useful for many applications including PL control and nonlinear effect.<br/>Keywords: Anapole, Slot effect, Metasurface, Dielectric, Field enhancement