## 3 Results
### Self-Organized Nanopatterning
This effect can be ascribed to the Ti layer presence which helps in preserving smaller structures on the sample and decreases coarsening effects. Similar considerations also hold for the other couples of samples prepared at different ion fluences \(F_{1}\) and \(F_{3}\), which allow us to draw some conclusions on the general trend observed in the morphological data. In Figure 3b, the averaged statistical parameters \(\sigma\) and \(\xi\) of all samples, extracted from their \(H_{\text{cf}}(r)\) functions, are summarized as a function of the increasing ion fluence. For what concerns the amplification of \(\sigma\) and \(\xi\) with increasing fluences, a clear trend can be inferred. RMS roughness \(\sigma\) and lateral correlation length \(\xi\), both increase by prolonging the ion dose during the A-IPL process. This increase of roughness at increasing ion fluences can be understood considering that the A-IPL process involves the use of an auxiliary gold evaporation source during the ion bombardment in order to prolong the shielding action of the Au nanowire stencil mask; in this way, the glass substrate is etched at deeper levels. This is confirmed by the strong increase of \(\sigma\) evident in Figure 3b, where points at a fluence lower than \(F_{1}\) = 1.6 \(\times\) 10\({}^{19}\) ions cm\({}^{-2}\) are extracted from Au NWs mask topographies. The increase in lateral correlation length at higher fluences can be ascribed to the coarsening of the smaller scale spatial modulations due to the ion and thermally activated glass relaxation. In this way, ion fluence is an experimental parameter which gives us the opportunity to easily tune the morphology of nanostructured glasses and hence their optical properties. On the other side, we also highlight that, for each explored ion fluence, the Ti adhesion layer allows us to decrease the average lateral scale and vertical extension of the nanostructures, expressed respectively by the statistical parameters \(\xi\), and \(\sigma\) (red open squares and triangles in Figure 3b). In particular, for the lowest ion fluence \(F_{1}\), the lateral correlation length of Sample 1-T measures 105 \(\pm\) 11 nm; this value is compatible with the one registered on the Au ripple pattern before its decomposition in nanowires [18]. In the early stages of the A-IPL process the adhesion properties of  
Figure 3: (**a**) One dimensional power spectral densities (1D-PSD) functions extracted from topographic AFM data of sample 2 (blue) and 2-T (red) are shown on log-log plot; (**b**) Averaged values of the RMS roughness \(\sigma\) (triangular symbols) and lateral correlation length \(\xi\) (square symbols) extracted from Height-Height correlation function \(H_{\text{cf}}(r)\) are plotted against the ion fluence. The samples prepared without Ti adhesion layer are colored in blue, the others in red. the Ti layer are thus capable of stabilizing the Au nanowires stencil mask, preserving the small features present on the pristine Au ripple pattern. The main physical mechanism is thus related to the inhibition of dewetting of the Au film once the continuous layer breaks up into an array of nanowires. The dewetting process is boosted by the hyperthermal energy deposition following ion impact which favours uphill mass transport of Au, thus exposing large portions of unshielded glass to harsh ion etching. This effect is manifested when the A-IPL process is prolonged at higher ion fluences \(F_{2}\) and \(F_{3}\), where the Au mask without Ti adhesion layer undergoes strong dewetting and allows the formation of large volume structures in the glass as a consequence of coarsening. In the presence of Ti instead, the pristine pattern of the Au continuous film is preserved and coarsening of the glass structures is prevented, resulting in more ordered and narrower glass ripples. In this picture, the Ti adhesion layer thus represents a further experimental parameter (in addition to ion fluence), which allows to control the morphology of the nanopatterned substrates prepared by ion sputtering in the A-IPL configuration.