## 3 Results and Discussion
### Optical Absorption Spectra  
Optical absorption spectra measured for all samples were fitted using Mie theory, as implemented in MieLab software [50]. Measured spectra (scattered points) and their corresponding fits (lines) for each ablation condition are shown in Figure 3. As can be seen, the quality of the obtained fits is very good. Ag NP concentration for the samples fabricated at 19.1 and 25.5 J/cm\({}^{2}\) in CTAB and PVP was so large that it was not possible to measure the optical spectra due to saturation of the spectrometer at the LSPR wavelength. Therefore, we had to dilute the original samples in order to measure the spectra; i.e., the concentration calculated for those samples was heavily underestimated at first. However, the calculated concentration was corrected by considering the total volume of solvent added, in order to calculate the actual concentration produced by the ablation process. The LSPR peak around 400 nm--typical from silver spheres--is clearly observable in all cases. It is also evident from the optical spectra that laser fluence did not affect the LSPR position, indicating that highly-symmetric nanostructures were produced for all fabrication conditions. Except for pure water (where they are not available), mean diameter and standard deviation of the size distribution obtained from the TEM micrographs were used as initial conditions for the fit of the optical spectra. Mean diameter barely changed during the fit, but the standard deviations obtained from the fits were slightly different than those obtained from the statistical analysis of the TEM images. Moreover, the fits of the optical spectra allowed us to calculate the concentration of the dispersed Ag NPs. Figure 4 shows the values of the mean diameters, together with the standard deviations (depicted as error bars) and Ag NP concentrations for each sample and fluence. The same information is shown numerically in Table 1. As we can see in both Figure 4 and Table 1, the addition of a stabilizer molecule had a direct effect on the nanocrystal size and concentration. Larger diameters (40 nm) were obtained for Ag NPs dispersed in pure water, and diameters were reduced to 34 nm for CTAB solution and to 21 nm for PVP solution. This is clear evidence of the effect of the stabilizer molecule in two different factors: (i) the growth of the nanocrystal that probably occurs at the expense of some silver atoms  
Figure 3: Optical absorption spectra for Ag NPs by laser ablation in (**a**) pure water, (**b**) CTAB solution, and (**c**) PVP solution as a function of laser fluence as indicated. Points refer to the experimental measurement, and straight lines refer to the fitted spectrum. dispersed in the solution by the plasma plume; and (ii) the reduction of agglomeration. Both effects were already apparent using CTAB, but were dominant when PVP was used as stabilizer. It should be noted that the nanoparticles obtained in this work with fs pulses were similar but slightly smaller than those fabricated using either ps [22] or ns [13] pulses. On the other hand, the influence of the solvent over the standard deviation was far less important. There was a slight reduction of the standard deviation for the samples containing CTAB and PVP, but in all cases, the standard deviation was close to 16 nm. The stabilizer molecule also affected the Ag NP concentration by increasing it, which seems to be a direct effect of the stabilization of smaller particles. Increasing laser fluence also generated higher concentrations, in all likelihood because the ablation rate was raised. Calculated concentrations for pure water vary from about \(2\times 10^{10}\) to \(2\times 10^{11}\) particles/cm\({}^{3}\) for fluences from 10.2 to 25.5 J/cm\({}^{2}\). For the same laser fluences, the concentration increased from about \(9\times 10^{10}\) to \(2\times 10^{11}\) particles/cm\({}^{3}\) and from about \(2\times 10^{11}\) to \(2\times 10^{12}\) particles/cm\({}^{3}\) when CTAB and PVP were added, respectively.