The significance of a fluence threshold for ultraviolet laser ablation and etching of polymers

The minimum fluence at which a laser pulse (FWHM 20 ns; 248 nm) causes a change in the surface of a film of polyethylene terephthalate (=PET) or polyimide (KaptonTM) in air has been measured by probing the surfaces with visible laser pulses of <1 ns at delay times of 10-10 000 ns. At fluences at which a single pulse left a permanent etch pit, the probe pulse showed an intense and rapid (<60 ns) darkening of the surface which may be attributed to the scattering of the beam by gas bubbles at the polymer surface. In 100 ns, a blast wave was visible which could be tracked for over 1000 ns. At the end (∞ ns), the polymer surface was not darkened but an etch pit was apparent. Progressively decreasing the fluence showed that even at fluences <0.025 J/cm2 for PET and<0.050 J/cm2 for Kapton, a single pulse transiently blackened the surface but did not leave an etch pit. The threshold for the ablative photodecomposition of these polymers appears to lie at a value of the fluence that is well below the threshold for measurable etching by a single laser pulse.