Tall asymmetric nanodimer plasmonic metasurfaces with an accessible and tunable hotspot for sensing applications

Plasmonic metasurfaces serve as a cornerstone for advanced nanophotonic applications, yet conventional planar meta-units frequently suffer from some detrimental effects of the substrate. Despite the greatly enhanced electromagnetic field, the hotspot is tightly confined to the metallic surface or within the tiny gap region of the metallic dimer nanostructure. To overcome these limitations, we propose and numerically investigate a metasurface composed of tall asymmetric gold nanodimer arrays. The nanodimer consists of two nanopillars of different heights. By breaking the out-of-plane symmetry, our plasmonic metasurfaces can generate a prominent toroidal dipole-based surface lattice resonance (SLR) mode with a high total quality factor and Fabry–Perot mode with a high radiative quality factor in the mid-infrared spectral range. Moreover, the position and strength of the hotspot can be manipulated in the out-of-plane direction. By optimizing the geometrical parameters, the SLR mode yields a unique, to our knowledge, combination of a high-quality factor ( Q =808.7), a large effective mode volume ( V eff =4.83×10 −4 λ 3 ), and a significant field enhancement (| E |/| E 0 |=70.6). These superior modal properties translate into exceptional refractive index sensing performance, with a high sensitivity of 3570 nm/RIU and a figure of merit (FOM) of 811.4. This paper presents a new, to the best of our knowledge, paradigm for designing three-dimensional asymmetric nanodimer plasmonic platforms that simultaneously offer high-quality factor and large, accessible interaction regions, paving the way for high-performance sensing and spectroscopic devices.