## 3 Results  
### Self-Organized Nanopatterning  
Light manipulation at the nanoscale relies on the ability to control morphological parameters of nanostructures. In our case, the nanofabrication process exploits in the first steps a self-organized array of Au nanowires which acts as a stencil mask. Under grazing incidence ion irradiation, an initially flat Au film develops a periodic ripple undulation at the metal-vacuum interface (Figure 1d). The ripple amplitude increases with ion dose while the residual thickness of the Au film decreases. When the bottom of the ripple valleys reaches the underlying glass, a disconnected array of nanowires is thus formed (Figure 1e). It acts as a stencil mask for etching the underlying substrate at later stages. Representative AFM topographies of Au nanowire stencil masks prepared either with (Figure 1a) or without (Figure 1b) a Ti adhesion layer interposed at the Au/glass interface show strong differences in statistical height; RMS roughness (\(\sigma\)) decreases from \(\sigma\) = 17 \(\pm\) 2 nm to \(\sigma\) = 11 \(\pm\) 2 nm when a Ti layer is employed. One can also notice that the longitudinal coherence and the order of the ripples are improved in nanowires synthesized with Ti layer and their lateral size is narrower, as shown by the topographic profiles superimposed in Figure 1c. In order to quantitatively describe the role of titanium in the pattern formation process, we analyzed AFM morphological data in the framework of dynamic scaling theories, recurring to second order statistical quantities [26; 27]. In the case of closely packed 1-dimensional nanostructures formed by IBS, one can easily derive the lateral scale of the structures from the lateral correlation length parameter, \(\xi\), extracted from the Height-Height correlation function \(H_{\text{cf}}(r)=\)\(<[h(x+r)-h(x)]^{2}>\). \(H_{\text{cf}}(r)\) has been systematically calculated for every AFM topographic line profile \(h(x)\), acquired along the fast scan direction which was kept orthogonal to ripple's long axis. The data were cumulated and averaged over different AFM images acquired in equivalent areas in order to decrease statistical errors. According to dynamic scaling theories for small values of the lateral displacement \(r\), \(H_{\text{cf}}(r)\) follows a power law, while for large displacement values, \(H_{\text{cf}}(r)\) tends to a saturation value \(2\sigma^{2}\), where \(\sigma\) is the RMS roughness [26; 27]. The asymptotic break between the two regimes of \(H_{\text{cf}}(r)\) allows us to determine the statistical lateral correlation length, \(\xi\), i.e., the distance r within which the surface height fluctuations are correlated. For distances \(r>\xi\), the height variations are instead spread randomly [26; 27]. In Figure 3b we report the lateral correlation length \(\xi\), extracted from the \(H_{\text{cf}}(r)\) as a function of the increasing ion fluence. In the case of Au nanowires mask it reads 95 \(\pm\) 11 nm, while in presence of Ti this figure is reduced down to 65 \(\pm\) 11 nm. The morphological differences introduced by Ti adhesion layer can be understood if we consider the role of Ti as an inhibitor of dewetting processes, either thermally activated or ion induced, thanks to the chemical and metallic bond of Ti with glass and Au respectively. Once the Au nanowire array is formed, it acts as a stencil mask which guides selective etching of the underlying substrate in correspondence to the disconnected gaps. At this stage of the process the substrate is heated to 700 K and an auxiliary Au atom source is switched on (sketch in Figure 2d). Figure 1: Atomic force microscopy (AFM) topographies (4 μm \(\times\) 4 μm) of Au nanowires stencil mask respectively without Ti adhesion layer (**a**) and with Ti (**b**) (vertical scale 70 and 124 nm); (**c**) Selected line profiles form (**a,b**); (**d**) Sketches undulated Au film prior to disconnection; (**e**) Au nanowire stencil mask after disconnection.