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M. Rafique, S. Bashir, A. Ajami, W. Husinsky:
"Nonlinear absorption properties correlated with the surface and structural changes of ultra short pulse laser irradiated CR-39";
Applied Physics A: Materials Science & Processing, 100 (2010), 4; 1183 - 1189.

We have investigated femtosecond laser irradiation
effects on the surface topography, structural changes
and nonlinear absorption properties of CR-39. For this purpose,
a CR-39 target was exposed in air to 25 fs, 800 nm Ti:
sapphire laser radiation at fluences ranging from 0.25 J cm−2
to 3.6 J cm−2. The surface of irradiated CR-39 probed by
an Atomic Force Microscope (AFM) exhibits the formation
of several topographical structures, like bumps, explosions
and nano cavities. Raman spectroscopy is performed
to explore chemical and structural modification of the irradiated
target. The spectroscopy reveals changes such as
cross linking, bond breaking, formation of new bonds etc.
in the fundamental structure of the polymer after irradiation.
In order to establish a correlation between morphological
and structural changes with the changes in the nonlinear
absorption of the irradiated CR-39, a Z-scan technique
was employed. A comparison of experimentally obtained
data from Z-scan measurements with our calculations predicts
the dominance of three-photon absorption in the case
of pristine CR-39, whereas for irradiated targets concurrence
of three- and two-photon absorption is probable. Nonlinear
absorption increases with increasing laser fluences and is
well correlated by surface and structural changes revealed
by AFM and Raman spectroscopy.