## 3 Results and Discussion
### Morphology of the Laser-Processed Surfaces  
Prior to laser structuring, the sample was mechanically polished to obtain a smooth and clean surface with no inclusions or surface structures as seen in the SEM image in Figure 2 (labelled Ti). Then, different regions on the CP Ti sample were laser-structured with a pulsed picosecond laser to generate micro-/nanostructures with different input laser processing parameters. During laser processing of any material, unique surface structures referred to as laser-irradiated periodic surface structures (LIPSSs) [18] are generated when the ablation fluence just exceeds the ablation threshold [31]. The removal of materials in pulsed laser processing is facilitated by a single Gaussian-shaped pulse. The pulse energy E, and by extension average laser power P, of a single Gaussian laser pulse are associated with the peak laser fluence F\({}_{\text{P}}\) through the relationship given in (1):  
\[\text{F}_{\text{P}}=\frac{2\text{E}}{\text{A}}=\frac{2\text{E}}{\pi\omega_{0} {}^{2}}=\frac{2\text{P}}{\pi\omega_{0}{}^{2}\text{f}} \tag{1}\]  
where A is the focused area on the surface of the substrate, \(\omega_{0}\) is the spot radius and f is the repetition rate. The surface morphology of the regions on the laser-structured sample is shown in Figure 2. Under laser structuring, the CP Ti material undergoes structural changes, forms  
Figure 2: SEM micrographs of ps laser-structured CP Ti surfaces showing LIPSS produced on regions A1, B1, C1, D1, and E1 using laser fluence of 0.2 J/cm\({}^{2}\), 0.5 J/cm\({}^{2}\), 1 J/cm\({}^{2}\), 1.2 J/cm\({}^{2}\), and 1.5 J/cm\({}^{2}\), respectively. surface structures, and becomes rough. Figure 2 shows SEM images at the surfaces of the laser-structured CP Ti after laser ablation with ps laser pulses at various laser fluences of 0.2, 0.5, 1, 1.2, and 1.5 J cm\({}^{2}\) at regions A1, B1, C1, D1, and E1, respectively. A 100 mm/s scanning velocity, 5 \(\upmu\)m hatching distance, and single scanning pass were used to process these five regions. The morphology of the surface nanostructures was determined using SEM without encountering any charging effects. This means that the oxide layer on the surfaces of the laser-structured regions was thin and, hence, could not have the charging effects normally encountered with thick titanium oxide [27]. At these regions, the PO and the LO were 93% and 80%, respectively [32], and this ensures that the laser pulses remove the material to form a shallow square cavity. At these five regions, the effective pulse number, N\({}_{\text{eff}}\), per focussed diameter (N\({}_{\text{eff}}=\frac{\text{D}\times\text{f}}{\
abla}\)) which facilitates the ablation process was 15 pulses [32]. As shown in Figure 2 the ablated part on region A1 has smooth and well-formed LIPSS. The LIPSSs formed had an average periodicity of approximately 462 nm and were oriented perpendicular to the laser beam polarization [18; 39]. Because the laser wavelength, \(\lambda\), is substantially longer than the periodicity, \(\Lambda\), (\(\lambda/2>\Lambda\)) of this type of LIPSS, it is called high spatial frequency LIPSS (HSFL) [39; 40]. The valleys of the LIPSS on region A1 (Figure 2, Inset on A1 region) are decorated by threadlike nanostructures that are oriented perpendicular to the HSFL. The ridges of the LIPSS on region A1 have been decorated by a few nanodroplets [24]. In Figure 2, the ablated part in region B1 has less smooth LIPSS than in the A1 region. These LIPSS have narrow valleys in comparison to those in the A1 region. The average periodicity of the LIPSS is 722 nm. This type of LIPSS is considered as low spatial frequency LIPSS (LSFL) because the laser wavelength, \(\lambda\) is longer than the periodicity, \(\Lambda\), (\(\lambda/2<\Lambda\)) of this type of LIPSS, and its orientation is perpendicular to the laser beam polarization [18; 39].