# Infrared Nanosecond Laser Texturing of Cu-Doped Bioresorbable Calcium Phosphate Glasses
## 3 Results and Discussion
### Glass Modification Threshold Test  
At first, the minimum average laser energy required to cause a visible modification on the glass substrate was determined, with the rest of the laser parameters remaining fixed. The MOPA fiber laser offers the advantage of delivering variable pulse width in the range of 4-250 ns. The lowest pulse-width of 4 ns was chosen in this work to minimize thermal-based effects such as the Heat Affected Zone (HAZ) in the laser-modified region [32]. The pulse repetition rate and scan speed were fixed at 20 kHz and 3 mm/s, respectively. Figure 2: UV-Visible absorption spectra of undoped and Cu-doped calcium phosphate glasses. Interestingly, the commonly observed laser-induced material removal did not take place in the case of copper-doped phosphate glasses. Figure 3a,b shows two images taken at the same modification spot, but at different focal planes. Upon focusing between the reference glass surface plane and the laser modification line, it is possible to see that the generated features along the laser scan line are in the form of protrusions above the reference glass surface plane as confirmed by the profilometer analysis presented later. The formation of micro-protrusion features, as depicted in Figure 3c, has an inherent relationship to the Gaussian energy distribution of the pulse and the temperature coefficient of surface tension \(\mathrm{d}\gamma/\mathrm{d}T\) of the glass. The Gaussian energy distribution would generate highest temperature at the center of the molten pool. Laser pulse interaction at the surface would cause rapid vaporization of the material, inducing a recoil pressure. The commonly observed micro-concave features after laser ablation are attributed to this recoil pressure. On the contrary, micro-protrusion features were obtained in the current study. This could be explained by a positive temperature coefficient of surface tension, which would mean the highest surface tension would be present at the center of the molten pool [33]. The temperature gradient of melt pool as a result of the Gaussian energy distribution of the pulse would thus cause a radially inward flow of the melt owing to inverse Marangoni flow [34; 35; 36]. To finally result in the observed protrusion features, the radial inward flow must overcome the recoil pressure effect caused by rapid vaporization. This inward flow drives the upward expansion of molten glass at the center of the melt forming the microfeatures on rapid cooling, as depicted in Figure 3c. Figure 3: Images of laser modification at the same spot but different focal planes. **(a)** Image focused on the laser modification; **(b)** image focused on the unmodified glass surface plane. The scale bar represents 50 μm. **(c)** Mechanisms of protrusion formation: (i) laser energy transfer and melt formation; (ii) melt flow and upward expansion; (iii) micro-protrusion after rapid cooling. In general, for laser interaction with materials, controlled surface modifications can be obtained within a certain range of parameters. Especially for longer pulsewidths, i.e., nano, micro, millisecond, and continuous wave lasers, thermal effects play a major role due to the pulsewidth being longer than electron-phonon and phonon-phonon energy coupling time up to the order of few ps [37]. Such thermal effects are responsible for certain unwanted features such as HAZs and thermal-stress-induced cracks in glass materials. Therefore, it is important to properly define the laser parameters to minimize such unwanted effects as much as possible to obtain designable surface modifications. Laser parameters of increasing average power and scan speed were studied to understand their effect on the modified surface features. An increasing average power at a constant pulse repetition rate would imply greater pulse energy. Since the ns pulsed modification is a thermal process, greater pulse energy would result in increased melt formation at the focus.