# 2D Dielectric Nanoimprinted PMMA Pillars on Metallo-Dielectric Films
## 2 Results and Discussion
The system heats up the substrate and the mold at the same time and the same temperature. Figure 3 shows the bird's eye and in-plane (inset) SEM image of the 2D array of PMMA pillars fabricated with \(p=600\) nm and diameter of about 250 nm when temperature and pressure were set equal to 180 \({}^{\circ}\)C and 40 bar. By using the two optimized technologies, i.e., the sputtering and the NIL, we easily realized dielectric gratings deposited on AlN/Cu slabs that support optical resonances. Figure 4a shows the cross-section of the fabricated bi-layer structure where the thicknesses of Cu and AlN are set equal to 100 nm and 150 nm, respectively, while the pillar diameter is equal to 250 nm. We covered the fabricated structures with a thin gold layer (thickness equal to about 15 nm) deposited by means of a process of 40 s sputtering with a gold target in order to inspect them by means of the SEM. Figure 1 shows the areas covered by gold at the bottom and on the top of the periodic pillars pattern. Figure 4b shows the SEM picture of the 2D pillar array on the multi-layer structure with the same periodicity. All the sizes comply with the design providing the right thicknesses of Cu and AlN (17 nm and 33 nm, respectively). It is worth stressing that the SEM image also determined the thickness of the PMMA buffer layer equal to about 20 nm (Figure 4c). All the fabricated devices, with and without gold, were optically characterized by means of an ad hoc setup, where the reflected light by the sample at normal incidence was analyzed by an integrated spectrometer [9; 10]. Figure 5 shows the comparison between the reflection spectra of both configurations without and with the gold layer, for the bilayer (Figure 5a) and the multilayer (Figure 5b), respectively. The plots reveal that, when the gold layer is taken into account, the resonance dip shifts from 629.8 nm to 634.9 nm (bilayer) and from 638.5 nm to 643.3 nm. Therefore, the shift is almost the same (5.1 nm vs. 4.8 nm) for both configurations. Conversely, the reflectance at the dip minimum drops from 49% to 21% (difference about 28%) and from 33% to 29% (difference equal to 4%) for the bilayer and multilayer, respectively. It is worth noting that the full width at half maximum (FWHM) slightly increases due to the presence of the plasmonic material for both configurations. In particular, the FWHM of resonance for the bilayer configuration changes from about 4 nm to 7 nm while it varies from about 5 nm to 8 nm in the case of the multilayer configuration when the gold layer is added. Finally, if we consider the reflectance in the wavelength range where the structures act as mirrors, we  
Figure 3: Bird’s eye SEM image of the fabricated device; (**inset**) in-plane detail of the fabricated 2D polymethyl-methacrylate (PMMA) plasmonic nano-pillar array. The measured pillar height is corrected for the tilt. can verify that the values drop about 20% and 8% for the two cases, respectively, leading to a reduction of a factor of 2.5. Considering the absorption as A = 1 - R, it is possible to conclude that the control of the absorption is improved by exploiting the metallo-dielectric slab. It is also worth highlighting that by just adding a thin gold layer it is possible to increase the total absorption. Indeed, the gold layer introduces more optical losses due to the plasmonic nature of the Au film. This is evident from the experimental results reported in Figure 5.  
Figure 4: SEM images related to the cross-section of (**a**) bilayer and (**b**) multi-layer designs, respectively. Cu, AlN layers and PMMA pillars (**black region**) while the grey area around pillars is a Pt deposition used to protect the sample while milling with FIB. (**c**) Cross-section of the bilayer sample revealing the PMMA buffer layer thickness equal to about 20 nm. Figure 5: Measured reflectance spectra of both configurations without (**solid blue line**) and with a gold layer (**dashed red curve**), for (**a**) the bilayer and (**b**) the multilayer, respectively.