# Laser Processing of Intraocular Lenses
## 3 Results
### Fabrication and Characterization of Femtosecond Laser-Induced Micro-Patterns  
A homogenous ablated area was formed on the central region of the PMMA IOL, as shown in Figure 3a,b, following irradiation with a pulse energy of 0.8 uJ and repetition rate of 100 kHz. Additionally, another ablated area was formed on the off-centered, curved side of the lens, as shown in Figure 3c,d. These areas covered a rectangular area of 100 um in width and 200 um in length and were created with a continuous scanning process without refocusing the objective lens. The lateral separation between each parallel scan was set at 4 um, a value that approaches the theoretical beam spot size of the lens (3.6 um). The intention was to achieve an effective overlap between parallel scans for a uniformly modified surface. For each line scan, the translation was set at 200 um and the scanning speed at 10 um/s. The ablation process on the PMMA IOLs results in the formation of a symmetrical region around the focal spot, overall displaying the characteristic roughness and porosity of a PMMA laser-ablated surface [25; 26]. The laser processing parameters were selected in order to prevent excessive damage on the surface of the PMMA IOL. It can be observed that when patterning at the off-centered, curved side of the IOL, due to varying focus conditions, there are residual threads of removed material, leading to incomplete ablation and increased surface roughness. However, our findings highlight the critical influence of precise focus conditions on the patterning mechanism and the consistency of the ablation procedure. By further optimizing the linear patterns' overlap and the writing depth, the surface uniformity and the roughness of the patterned area could be further improved where needed in real applications. Figure 2: Optical transmission spectra for PMMA and acrylic IOLs. On the off-centered, curved side of the acrylic IOL, we again created a 100 \(\upmu\)m in width and 200 \(\upmu\)m in length rectangular area using exactly the same exposure conditions as those employed for the PMMA IOL. This process results in the creation of structured regions, as illustrated in Figure 4a-c, which portray the ablated area at various magnifications. The SEM analysis reveals several notable features such as an expanded width in a single line scan, as presented in Figure 4d; the presence of molten material; and the formation of micro-cavities. The observed micron-sized cavities likely result from gas release during the thermal degradation of the acrylic material. These findings strongly indicate localized thermal modification in the patterned area, representative of a photo-thermal ablation process. Although heat accumulation effects with femtosecond lasers typically manifest for repetition rates higher than 200 kHz or even 1 MHz [27; 28], the response depends on the thermal properties of the target material. Properties including thermal diffusivity, melting point, and heat capacity play a fundamental role in determining its response to the intense heat generated during laser ablation. For example, the thermal diffusivity of PMMA and other acrylic polymers can be influenced by the molecular weight or the presence of additives in the polymer [29]. These factors can significantly affect the ablation process, potentially altering the material's behavior under laser exposure. The peak laser fluence is determined, considering a Gaussian beam, using the equation  
\[F=E_{p}/\pi(d/2)^{2}, \tag{2}\]  
where \(E_{p}\) represents the pulse energy set at 0.8 \(\upmu\)J and \(d\) is the focused beam diameter. The peak fluence used for the micromachined areas was 8 J/cm\({}^{2}\). This value exceeds the ablation threshold for both PMMA and acrylic materials. We also determine the effective  
Figure 3: SEM images (top view) of the ablated areas. (**a,b**) At the center of the PMMA IOL; (**c,d**) on the off-centered, curved side of the PMMA IOL.