Field enhancement effect in graphene-based terahertz nano metasurfaces

In the paper, we discuss the field enhancement effect in terahertz nano metasurfaces. The unit cell of the metasurfaces consists of a metallic split ring resonator (SRR) connecting with a wire. When the gap of SRR varies from micron-scale to nano-scale, the field enhancement factor in the gap achieves an order-of-magnitude increase in nano metasurfaces compared to that of micron metasurfaces. We then apply the nano metasurfaces to electric field sensing by assembling a layer of graphene film. In the simulation, the conductivity of graphene is tuned by varying the scattering time (relaxation time) corresponding to the varying external voltage. Compared to the structure without graphene film, the transmission of the graphene-based metasurfaces will be modulated by graphene conductivity. And the conduction effect of the graphene-based metasurfaces with nanogap under the same voltage is much better than that of the structure with micron gap, due to the extreme high field enhancement of the former. Based on this study, we can further optimize the nano metasurfaces for high sensitivity sensing, which can be applied in biological/chemical sensors or nonlinear devices.