Narrow resonance line-widths and high figure of merits via composite nanopillar dimer array sensors

The Ag/Si/SiO2 composite nanopillar dimer array sensor is put forward to form a surface lattice resonance (SLR) with strong intensity and narrow resonance line-width. In the Ag/Si/SiO2 composite nanopillar dimer array, Ag nanopillar dimers sit on Si nanopillar dimers, and Si nanopillar dimers sit on SiO2 nanopillar dimers, which sit on a quartz substrate. Simulated results demonstrate that the line-width can be as narrow as 2.1 nm. This is mainly attributed to the coupling between the combination of single Ag nanopillar dimers' local surface plasmon quadrupole (Q) resonance and Si nanopillar dimers' Mie resonance with the array's diffraction waves. However, for the Ag/Si/SiO2 composite nanopillar array (without dimers), the SLR intensity is too weak for the sensing applications. Compared to the Ag/Si/SiO2 composite nanopillar array, the superiority of the Ag/Si/SiO2 composite nanopillar dimer array is that the near-field coupling in the dimer can further improve Ag nanopillar dimers' Q resonance and Si nanopillar dimers' Mie resonance. Therefore, SLR intensity can be significantly enhanced with only a slight increase in line-width, thus the sensing detection range is wider. The array's period, the height and diameter of Ag/Si/SiO2 nanopillars have notable effects on the SLR. Benefiting from the narrow line-width, the figure of merit (FOM) can reach a value of 226. This work is important to improve the optical sensor based on SLRs.