▎ 摘　　要

The ubiquitous property of metamaterials to confine energy at deep-subwavelength scale paves the way to leverage strong light (or terahertz)-matter interactions towards the development of efficient, state-of-the-art sensing technologies at the terahertz (THz) frequencies. In two-dimensional active metamaterials, for instance, graphene metasurfaces; THz-matter interactions are largely governed by the near-field coupling, where strong field confinement is predominantly achieved within very small areas (compared to total device area), viz, in split gaps, narrow apertures etc. Here, we demonstrate a polarization insensitive broadside coupling mechanism in graphene- metamaterials, which is beneficial for realization of miniaturized, compact, ultra-sensitive THz thin film sensors with substantially large effective areas for strong THz-matter interaction. Proposed structure entails ultrathin layer of dielectric spacer (cavity) sandwiched between two "plus" shaped sub-wavelength graphene resonators. In order to investigate the sensing capabilities, obscure materials are placed in ultrathin cavity region to detect its refractive indices. A record high refractive index sensitivity of around 2.8 THz/RIU substantiates the utility of introducing the concept of broadside coupling in graphene metamaterials. Further, we employ the Lorentz oscillator based coupled mode theory in order to elucidate the numerical outcomes.