# Laser Assisted Size Reduction of Gold (Au) Particles onto a Titanium (Ti) Substrate Surface
## 4 Discussion  
The assessment of laser beam impact on the gold coated material was done using the whole range of the laser powers available in the A-355 laser micromachining system  
Figure 11: Histograms of particles distribution: (**a**) 4 mW; (**b**) 15 mW; (**c**) 16 mW. (Oxford Lasers, Didcot, UK). The treatment was performed using both the 400 Hz approach with a feed rate of 1 mm/s and the \"single laser irradiation\" method, which investigates the effects of a single laser pulse at a given power. When comparing the graphs of the two approaches, a certain analogy can be seen, but it is more evident when processing at 400 Hz (above 13.2 mW the groove depth increases exponentially). The recorded results for both power-depth relationship curves allowed us to select the optimal laser power range at 13.2, 17.6 and 22 mW. Additionally, in selecting the optimum parameters, it is important that even at the lowest power tested, the beam penetrated through a 100 nm thick layer of sputtered gold. Moreover, light microscopy images show an interesting tendency for particles to agglomerate outside craters, an agglomeration that is not visible in the case of a single irradiation. Based on the microscopic images obtained and EDS maps, the presence of nano-size gold particles with a distribution dependent on the crater distance after the beam passage can be observed (Figure 9). The geometric character of the crater recorded by 3D analysis with a confocal microscope is typical for a single interaction of the Gaussian beam with matter as has been shown in numerous literature studies [27; 28; 29]. It can be observed that the debris which gathered around the ablation crater was a combination of re-solidified droplets of ejected molten material, globules, and clusters of small particles. This is because the laser used does not achieve pure ablation--the average energy flux density does not exceed \(3.2\times 10^{7}\) W/m\({}^{2}\). This relationship is described and confirmed by [PERSON] et al. [30]. They defined that if the energy flux density is less than \(10^{12}\) W/m\({}^{2}\), particles decrease their size in a mechanism following a strict order: heating-melting-evaporation. So, if the evaporation of the material is partial, the majority is melted. Following the statement of [PERSON] et al. [31] that size reduction of the particles occurs in a layer-by-layer mechanisms based on bimodal particle distribution, the most important was to assess the size and amount of particles present in irradiation area already after one single pulse. Observation of the bimodal distribution of particle sizes that are slightly smaller and much smaller than the original ones allowed [PERSON] to conclude that the irradiation by picosecond-laser pulses is the most efficient way to reduce the Au NPs size. Later, [PERSON] carried out the calculations to examine the melting and size reduction of AuNPs, stating that photothermal mechanisms prevails at low laser intensities, that occurs with best efficiency in 532 nm nanosecond lasers in comparison with wavelengths of 1064 and 355 nm, but he gave no experimental evidence for that finding. The 355 nm picosecond laser has been found efficient for splitting Ag NPs [32] and Au NPs in aqueous sodium dodecyl sulphates, as the picosecond excitation-relaxation electronic interband enables a faster absorption of a new photon that the conduction-band electrons can absorb. What is more, positively charged Au ions occurring during interband excitation allow the thermionic emission of electrons, which is responsible for the highly efficient splitting of the Au NPs [33; 34]. Taking into account our previous study on Au particles infused into polyether ether ketone (PEEK) polymer [10], and the fact that picosecond laser tends to be the most suitable for an efficiently to inducing size reduction [31], single laser irradiation should be analyzed at the first step in gold reduction trials. As shown in Figure 9, the scale of reduction of the gold layer to particles occurred to a very large extent in the material we studied.