# Self-Organized Conductive Gratings of Au Nanostripe Dimers Enable Tunable Plasmonic Activity
## 2 Results and Discussion
### Plasmon Hybridization in Nanostripe Dimer Arrays  
The control enabled by the self-organized nanofabrication process allowed us to develop a multi-step approach for increasing the complexity of the plasmonic nanoantennas, while still preserving the large area homogeneity of the sample. The isolated nanostripes have been vertically stacked, forming tilted nanostripe dimers which feature plasmon hybridization due to the near field coupling between the individual nanoantenna elements. This has been achieved by selectively coating the tilted Au nanostripes with a SiO\({}_{2}\) layer confined at a glancing angle (Figure 3a) by RF magnetron sputtering from an SiO\({}_{2}\) target, followed by a further Au deposition step. Under this condition, laterally disconnected Au-SiO\({}_{2}\)-Au nanodimers are formed in register with the underlying nanopatterned template, as demonstrated by the SEM image of Figure 3b. The top-view SEM image (Figure 3c) clearly shows that the lateral disconnection of the nanodimers is still preserved, thus obtaining homogeneous quasi-1D arrays over large areas. Such tailored nanoscale morphology strongly affects the optical properties of these templates, as shown in Figure 3d. The plot represents the evolution of the optical transmittance detected for TM illumination during the growth of dimer nanoantenna arrays (\(w\) = 105 nm, thickness of the Au and of the SiO\({}_{2}\) layer reading \(h\) = 30 nm and \(d\) = 44 nm, respectively). The strong dichroism and the plasmonic excitation characteristic of the single nanostripe configuration (black spectrum) is preserved after the silica growth (blue spectrum), which is slightly red shifted from a 560 nm to 591 nm wavelength, due to the increase of the average refractive index of the dielectric medium. Since by sputtering deposition we confined a non-stoichiometric SiO\({}_{\rm x}\) film, the effective refractive index of the layer can be calculated considering the measured optical absorption of the bare SiO\({}_{\rm x}\) film. In the present case, a refractive index of about 1.75 has been evaluated. Interestingly, a splitting of the plasmonic resonance, i.e., plasmon hybridization, is observed when the second Au nanostripe layer is deposited (red curve). A blue-shifted electric dipole (ED) mode is here excited at 550 nm, while a red-shifted magnetic dipole (MD) mode is detected at 700 nm. The extinction spectrum shows a narrow-band ED mode, while a broadening of the MD mode is observed due to the high sensitivity of this resonance to slight variations of the parameters from the individual nanostrip dimers of the array (which is inherent to the self-organization process) [31]. The figure of merit of the hybridized configuration in the near field regime has been evaluated by FEM calculations, revealing a strong sensitivity in the subradiant response of the nanostripe dimers. The electric near-field distribution at resonance is shown in Figure 3e,f for the case of tilted nanostripe dimers excited in resonance with the ED and in the MD mode, respectively. The near-field simulations clearly show a plasmonic near field enhancement in proximity of the nanostripe edges, with field enhancement factors \(|\)E/E\({}_{0}|\) at the single nanostripe edges amounting to about eight. In the case of the nanostripe dimers, we calculate \(|\)E/E\({}_{0}|\) of about 3.5 for ED mode excitation, while a strong enhancement of 19 is observed in the case of MD mode excitation. This characteristic feature clearly evidences the advantage of the nanostripe dimer configuration under MD mode excitation, in view of highly sensitive linear and nonlinear spectroscopies where superior plasmonic amplification factors are required. In order to prove the capability of the method to tailor on demand the resonant wavelength of the hybridized mode, FEM simulations have been performed either by changing the Au nanostripe aspect ratio _h/w_ in the dimer (Figure 4a) or by changing the thickness of the silica dielectric spacer.