# Laser Processing of Intraocular Lenses
## 3 Results
### Fabrication and Characterization of Femtosecond Laser-Induced Micro-Patterns
number of pulses delivered per surface area, calculated as 40 \(\times\) 10\({}^{3}\) for both IOLs, utilizing the formula  
\[N=RR\times d/s, \tag{3}\]  
where \(RR\) represents the pulse repetition rate, \(d\) is the focused beam diameter, and \(s\) is the translating speed of the stage. Following irradiation by the femtosecond laser, we performed depth measurements with the surface profilometer, along the short dimension of the ablated areas (100 \(\upmu\)m in width), to evaluate the material modification and the precision in the laser ablation process. As indicated in Figure 5, the ablation depth for the PMMA IOL ranges from 3 to 3.5 \(\upmu\)m in the area created at the center of the lens (blue line) and from 1 to 3 \(\upmu\)m in the area located on the off-centered, curved side of the lens (red line). In contrast, for the acrylic IOL, the measurement of the modified area located on the off-centered, curved side of the lens (black line) exhibits a deeper ablation depth of 20 to 30 \(\upmu\)m, as shown in Figure 5. In the off-centered modified regions of the lenses, the surface exhibits non-uniform modification depth, indicating variations probably due to the different distance from the beam focus and the absence of refocusing during the irradiation process. Further investigation into irradiation parameters with the femtosecond laser revealed that employing higher repetition rates, such as 1 MHz, in combination with the different materials under examination, resulted in a significant increase in ablation depth. This was evidenced by the observation of 50 \(\upmu\)m deep and 50 \(\upmu\)m wide ablation spots observed on PMMA IOLs, as shown in the measurements obtained with the surface profilometer in Figure 6. This increase can be attributed to the higher energy delivered to the target material per unit time, resulting in more substantial material removal per laser pulse or over a given  
Figure 4: SEM images (top view) of the ablated areas on the off-centered, curved side of the acrylic IOL. (**a–c**) Ablated area with different magnifications; (**d**) ablated line scan. time. The higher energy levels or repetition rates impart greater momentum to the laser-induced plasma, leading to increased material ejection and, consequently, deeper ablation depths. Additionally, the scanning speed is another parameter affecting the ablation depth. This observation underscores the significant influence of laser writing parameters [30] on the material processing characteristics, particularly in the context of the laser ablation of IOLs for applications such as surface modification. The further exploration of different irradiation parameters could potentially lead to achieving similar modifications on acrylic IOL to those achieved on PMMA IOL. The above findings suggest that the employed infrared femtosecond laser can be efficiently used for patterning in both PMMA and acrylic IOLs' surface with very good repeatability and resolution due to the consistent response of the material. Various femtosecond lasers have been employed in the past. For example, in [22], an 800 nm fs laser with much higher pulse energy up to 200 \(\upmu\)J, but with a low repetition rate of 5 kHz, was employed for writing linear structures with depths up to 5 \(\upmu\)m. Also, in [23], a femtosecond laser at 520 nm with much lower pulse energy of 1 nJ was employed for IOL but with discontinuous line patterning due to thermal effects attributed to the very high repetition rate of 63 MHz. Figure 5: Indicative depth measurements of the ablated areas after irradiation with the femtosecond laser. PMMA IOL: two ablated areas, at the center (blue line) and off-center (red line) of the lens. Acrylic IOL: one ablated area off-center (black line) of the lens. Figure 6: Depth measurement of an abated spot on PMMA IOL irradiated with the femtosecond laser at 1 MHz. In general, two distinct regimes of femtosecond laser writing or micromachining can be identified based on the pulse period relative to the time required for heat to diffuse away from the focal volume.