# Laser Processing of Intraocular Lenses
## 3 Results
### Characterization of Structural Modification and Chemical Composition
The spectra show narrow (compared to the PMMA spectrum) Raman bands in the 200-1650 cm\({}^{-1}\) range, the strongest of which are seen at 495, 621, 769, 825, 1002, 1031, 1202, 1303, 1417, 1446, 1584, and 1605 cm\({}^{-1}\) and are related to phenyl ring modes (621, 1002, 1031 cm\({}^{-1}\)) [39] or C-C or C-O stretch or alkyl group deformation modes in the polymer substance. The two strong and broader bands in the region 2800-3200 cm\({}^{-1}\) with maximal intensities at 2923 cm\({}^{-1}\) and 3057 cm\({}^{-1}\) correspond to a convolution of CH\({}_{2}\) and CH\({}_{3}\) symmetric and asymmetric C-H stretching modes in singly and doubly bonded C atoms, respectively. The peak at approximately 1730 cm\({}^{-1}\) depicts the C=O stretching mode of acrylates, analogous with the 1725 cm\({}^{-1}\) mode for PMMA. The absence of the C=C bond stretch signature at ca. 1640-50 cm\({}^{-1}\) in the pristine acrylic IOL material confirms the absence of residual monomers and its biocompatibility [37]. In the case of the PMMA IOL, a tiny component at ca. 1640 cm\({}^{-1}\) is hardly visible, revealing the presence of a low concentration of residual monomers. Figure 11: Raman spectra of PMMA IOLs before and after laser ablation with femtosecond 1064 nm and CW 405 nm lasers. (**a**) Regular; (**b**) confocal micro-Raman spectra. The Raman spectra of areas structured with the CW 405 nm laser show a low Raman intensity diminution and a moderate background photoluminescence without significant changes in the band positions when compared with non-processed areas. This Raman intensity diminution indicates that, even though the polymer structure appears to remain almost unaltered, the ablation process must be producing some photo-thermal modification. These results are generally in line with other works on similar materials [23, 40, 41]. The Raman intensity diminution was stronger in the confocal spectra (acquired with the \(\times\)100 lens); this indicates that, whatever the laser-irradiation-induced changes may be, they are localized closer to the surface, while the spectra acquired with the \(\times\)20 lens correspond to a higher depth of field and consequently probe deeper in the material where the laser irradiation has not penetrated. However, the laser-ablated areas structured with the femtosecond 1064 nm laser (0.8 \(\upmu\)J pulse energy, 100 kHz repetition rate) show a significant decrease in the intensity of the Raman peaks and a strong background photoluminescence. Defects, monomers, or other molecule fragments resulting from the degradation of the polymer network or backbone are at the origin of this background fluorescence either in the pyrolytic decomposition process or in the UV laser ablation process (see, e.g., [42]). A portion of this fluorescence may show up in the Raman spectra depending on which laser line is used for their acquisition. This observation is consistent with other studies [23], despite the different laser characteristics used in those studies (520 nm wavelength, 2 nJ pulse energy, and 63 MHz repetition rate) and even though the infrared 1064 nm irradiation photon energy is the lowest. Specifically, with confocal Raman (using the \(\times\)100 lens), carbonization (see the disordered/amorphous-graphite-like doublet at 1351 and 1596 cm\({}^{-1}\)[43], as in the case of above threshold irradiation of silicone-based hydrogel polymers [42]) is observed close to the surface layer for the acrylic material; this effect is not evident when the low magnification \(\times\)20 lens is employed. The bands at 1305 and 1345 cm\({}^{-1}\) do not show any broadening (at least as detected with the \(\times\)20 lens). On the contrary, the broadening of the bands at 1310 and 1347 cm\({}^{-1}\) in [23] has been interpreted as a laser-irradiation-induced photo-thermal damage and hence structural degradation of the acrylic IOL.