# 2D Dielectric Nanoimprinted PMMA Pillars on Metallo-Dielectric Films
## 1 Introduction  
The optical properties of diffraction gratings have attracted great interest for a wide plethora of applications across the electromagnetic spectrum spanning from photodetectors, THz spectroscopy, optical filters, photovoltaics, and color coatings [1]. One important class of optical resonators is based on guided-mode resonances (GMRs), where a grating is exploited to excite leaky modes in a guiding layer achieving absorption on a narrowband [2]. Several examples have been proposed in the literature showing metamaterials [3; 4; 5; 6], also combined with two-dimensional materials, such as graphene [7; 8; 9; 10; 11]. Some of these schemes use mirrors in order to obtain near-perfect absorption leading to a one-port device. Generally speaking, all these resonant structures are very sensitive to the geometrical parameters of the fabricated nanostructures, such as the sidewall of the features, and, thus, precise fabrication is indispensable to achieve the designed performance. This is critical when narrowband absorbers are considered. Consequently, the fabrication of high-quality optical gratings requires precise control of the geometrical parameters at the nanometer scale. In our previous works reported in Ref. [9; 10], we have shown the excitation of GMRs in dielectric gratings, based on polymethyl-methacrylate (PMMA) strips on Ta\({}_{2}\)O\({}_{5}\) slabs, that were realized by means of an electron-beam lithography system and were based on 1D periodic structures.  
In this paper, we propose an optimized nanoimprint lithography (NIL) protocol for the fabrication of a two-dimensional (2D) array of polymethyl-methacrylate (PMMA) nano-pillars deposited ondifferent sputtered configurations (bilayer and multi-layer) of copper (Cu) and aluminum nitride (AlN) slabs supported by a silicon dioxide (SiO\({}_{2}\)) substrate. Both the Cu/AlN bilayer and multilayer thin films were deposited by a sputtering technique, while the 2D sub-micron PMMA pillars were realized by nanoimprint lithography. The NIL technique has attracted great attention as an alternative nanopatterning technology and has found applications in many areas of nanoscale device fabrication ranging from more standard semiconductor devices [12; 13] to more unique applications in optics [14; 15] and plasmonics [16; 17; 18]. [PERSON] et al. [19] have demonstrated that using NIL with a high aspect ratio mold, it is possible to directly imprint flexible polymer pillars that can come into nanometer proximity at a predefined symmetry, thus, opening a new path for the design and fabrication of arbitrary geometries of plasmonic nanostructures. The precise and repeatable replication of nanoscale patterns [20] from a single high-resolution patterning step makes the NIL technique much more versatile than other expensive techniques such as electron beam or even helium ion beam lithography. Compared to other lithography techniques, NIL has the advantages of being high-throughput and low-cost.  
In particular, we report on the integration of NIL-based cylindrical pillars with a metallo-dielectric stack focusing on the optical behavior of the imprinted dielectric nanostructures. The fabrication parameters were studied and optimized. In order to verify the resonance behavior of the grating, we compare the performance of a bilayer (dielectric/metal or single-pair) configuration that supports GMRs with respect to a counterpart configuration where the dielectric slab is substituted by a multi-layer structure. We also investigated both configurations when a sputtered thin gold layer, introduced to avoid the charging effects during the scanning electron microscope (SEM) inspections, covers the dielectric pillar.  
These fabricated structures pave the way for the realization of robust photonic devices and robust/stable hybrid structures that can efficiently combine both photonic and plasmonic resonances, taking advantage of both peculiarities [21; 22].