## 3 Results  
For the fabrication process, the simple procedure was enough to create a surface nanostructure, which was then capable of Raman signal enhancement of the R6G spectrum via SERS effect. Through SEM, the formation of repetitive nanostructured ripples on most of the fabricated substrates was observed. An overview of the laser fluence and the number of overlapping pulses on LIPSS created on the Cu substrate covered by 200 nm Au thin film is presented in Figure 1. The effect of the laser fluence on LIPSS formation is visible on the horizontal axis. It can be observed that the ripples become more pronounced with increasing laser fluence. In addition, the effect of the number of overlapping laser pulses on LIPSS formation is presented in the same figure. At much higher laser fluence and larger numbers of overlapping pulses, the periodicity will vanish, as a large amount of material is ablated. A \"sweet spot\" for ripples formation can be found by considering the two variables. In order to obtain the surface-pattern period, a Two-Dimensional Fast Fourier Transform (2D FFT) from each SEM micrograph was performed. The 2D FFT image is shown on the right-hand-side of each SEM micrograph of Figure 1. Only the magnitude/intensity part of 2D FFT was used. The analysis performed on the high magnification SEM pictures revealed the period of the nanometric ripples, which was around 800 nm. The scale for the 2D FFT insets from Figure 1 is given in wave numbers, calculated with k\({}_{0}\) = 2\(\pi\)/\(\lambda_{0}\), where \(\lambda_{0}\) = 1064 \(\upmu\)m is the wavelength of the laser pulses. Figure 1: SEM micrographs (20,000\(\times\)) showing the influence of laser fluence and number of overlapping laser pulses on Laser-Induced Periodic Surface Structures (LIPSS) formed on Cu-Au. The insets show the intensity of the Two-Dimensional Fast Fourier Transform (2D FFT) transform and their scale is \(k_{0}\) = 5.9052 \(\mu m^{-1}\). (**a**) 0.6/cm\({}^{2}\), 360 pulses; (**b**) 1.2/cm\({}^{2}\), 360 pulses; (**c**) 2.4/cm\({}^{2}\), 360 pulses; (**d**) 0.6/cm\({}^{2}\), 65 pulses; (**e**) 1.2 J/cm\({}^{2}\), 65 pulses; (**f**) 2.4 J/cm\({}^{2}\), 65 pulses; (**g**) 0.6 J/cm\({}^{2}\), 36 pulses; (**h**) 1.2 J/cm\({}^{2}\), 36 pulses; (**i**) 2.4 J/cm\({}^{2}\), 36 pulses. From the experiments performed under various laser fluences and number of overlapping pulses it seems that the morphology of LIPSS is largely influenced by the laser energy density delivered on the unit of area. It is visible that the laser fluence and number of overlapping pulses should exceed the values used to produce (d) and (h) in order to obtain LIPSS. As shown in Figure 1d,g,h, both the laser fluence and number of overlapping pulses have their own threshold, which has to be passed in order to obtain defined LIPSS. At the higher values for both variables, the periodicity is affected, most probably by a different regime of laser ablation. The 2D FFT images reveal that these values must not pass the ones used to produce (b) and (f). The SEM image of one laser processed Cu-Au portion is presented below, in Figure 2a. A wide area of regular ripples was formed in the laser irradiated region, visible as a darker vertical stripe. EDX surface mapping analysis indicated the presence of small quantities of Au on the ripples area (Figure 2e) compared to the quantity of Cu. The more in-depth probing of the EDX technique goes well beyond the 200 nm thin film of Au, therefore even a small percentage is meaningful after laser processing. Additionally, XPS survey spectra shown in Figure 2b,d also revealed the presence of Au on the laser processed surface but, as expected, in considerably smaller amounts than the unprocessed surface. Due to the laser ablation, small amounts of Cu are also present on the unprocessed surface.