## 3 Results
Figure 2: SEM image (**a**) and EDX mapping (**c**) of a Cu-Au substrate produced by laser irradiation with 0.6 J/cm\({}^{2}\) laser fluence, 360 overlapping pulses; XPS Surveys spectra of processed (**b**) and unprocessed points (**d**); EDX spectrum of processed area (**e**). Furthermore, the XPS experiments revealed how the quantity of Au in the composition changes by varying the number of overlapping pulses or the laser fluence. In Figure 3, the intensity of the Au peak at 84 eV falls rapidly, both by increasing the number of overlapping pulses and by increasing the laser fluence. The point of zero laser fluence in the right hand panel represents the XPS intensity of an unprocessed area of Cu-Au. In Figure 4, an experimental characterization of the high purity R6G powder's Raman spectrum is presented. The high intensity peaks from literature can be observed [21; 22; 23; 24]. After having applied the built-in fluorescence correction software [25], the peaks are close to the background noise. It is also difficult to draw sufficient information from the Raman fingerprint regions. It is worth stating that, for R6G, increasing the laser power or the spectral data acquisition time causes both the Raman signal and the fluorescence signal to intensify without significant changes to their ratio. A time consuming method of increasing the signal-to-noise ratio is to irradiate the sample without gathering data and wait for the fluorescent response to lose some of the initial intensity. Further on, Raman signal amplification test results are presented below as direct comparison between different samples of Cu and Cu-Au substrates. In Figure 5, the corrected Raman spectra of the first analyzed substrates are presented. These are Cu and Cu-Au, which were produced with the  
Figure 4: Raman spectra of pure Rhodamine 6G solution (R6G) powder on glass after fluorescence correction. Figure 3: XPS Intensity variation of the 84 eV peak of Au on substrates produced by picosecond laser irradiation with either different laser fluences and fixed 65 overlapping pulses (**a**) or produced with different number of overlapping pulses and fixed 1.2 J/cm\({}^{2}\) laser fluence (**b**); red lines are B-spline interpolations. same laser fluence of 1.2 J/cm\({}^{2}\), but with different average numbers of overlapping pulses. On the left hand panel, the Cu-Au substrates appear to provide satisfying SERS amplification, considering that the amount of substance excited by the Raman laser is considerably smaller because of the lower concentration of the analyte (10\({}^{-4}\) M solution compared to 99.9% purity powder). For better visibility, the intensity of a single peak for each substrate was also plotted in Figure 5. It shows the Raman intensity of the most intense Raman shift value for R6G excited with 632.8 nm laser (caused by C-C and C-N stretching [21; 22; 23; 24]), i.e., the 1356 cm\({}^{-1}\) peak of each spectrum. The detailed analysis of the maximum peak of R6G from 1356 cm\({}^{-1}\) reveals that between 50 and 90 overlapping laser pulses is the optimum from the point of view of SERS effect. Judging by the visibly distorted ripples seen in Figure 1d,g,h, the slightly less powerful amplification at low numbers of overlapping pulses could be caused by the lack of well-formed ripples. For Cu, the higher number of overlapping pulses seems to provide worse enhancement. The laser fluence of 1.2 J/cm\({}^{2}\) might have been too high over the ablation threshold and so the variation of amplification is not significant when varying the number of overlapping pulses which produced the substrate. The difference between the Cu and the Cu-Au substrates peak intensities (i.e., a difference higher than an order of magnitude) generated by the variation of the number of overlapping pulses is not visible. In addition, higher laser fluences and numbers of overlapping pulses lead to the formation of oxide droplets, which do not increase the SERS amplification.