Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

ABLATION PROPERTIES OF FILOFOCON A
magnification optical images of unablated (left side of the image) and ablated (right)
areas in Filofocon A (Fig. 4.6(a)) and PMMA (Fig. 4.6(b)), for 30 pulses and a fluence
of 400 mJ/cm 2 . The different structures inside the ablated area (right part of each
image) are also present even with very low number of pulses.
(a) Filofocon A
(b) PMMA
Fig. 4.6. High magnification (100x) optical micrographs of Filofocon A (a) and
PMMA (b). In both pictures, the ablated zone (400 mJ/cm 2 , 30 pulses) is on the right.
The scale bar is 10 m.
AFM images (shown in Fig. 4.7) confirm that these structures, close to the
resolution limit of the camera of the optical microscope, are real physical structures
and not optical effects such as aliasing.
(a) Filofocon A
(b) PMMA
Fig. 4.7. AFM images of the ablated zone (400mJ/cm 2 , 30 pulses) in Filofocon A (a)
and PMMA (b). The scalebar is 2 m. The peak to peak depth (color scale range) is
300 nm.
4.5. DISCUSSION
We have studied the excimer laser ablation of Filofocon A and PMMA materials, in
the operational range of refractive surgery. These materials have been used in previous
studies in experimental models of corneal refractive surgery (Dorronsoro et al., 2006a,
Chernyak and Campbell, 2003, Dorronsoro et al., 2008a), although their ablation
properties were not well established.
Both materials show a large range of linear dependence between ablation depth
and number of pulses, for different fluences (see Fig. 4.2) as well as between ablation
rate (depth/pulse) and the logarithm of the fluence (Beer Lambert law, see Fig. 4.4).
We have measured optical and ablation properties of these materials at 193 nm
(including index of refraction and extinction coefficient, ablation threshold and
effective absorption coefficient) and investigated the structural changes in the material
following ablation. We have shown that incubation is not negligible for low fluences,
and that the ablation parameters change with the number of pulses, more so in PMMA
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