The dynamics of pulsed laser ablation of materials has been probed optically recently by high speed imaging of the substrate surface and of the blast wave generated by the process.1–3 In a preliminary study4 we reported that the optical pulse of a KrF excimer laser reflected from a polyimide surface is truncated once the incident laser fluence exceeds a threshold value, which is close to the threshold fluence for laser ablation. This result showed that for laser pulse widths on the order of nanoseconds, the onset of ablation does not depend on the intensity, but on the accumulated energy. In order to understand the mechanism of the transient decrease of reflectivity at the onset of ablation, we have now extended our preliminary experiments to include several polymers with various chemical structures and having a wide range of absorption coefficients and ablation thresholds fluences at 248 nm. Optical pulses from a uniform beam Lumonics HyperEX 460 excimer laser operating at the KrF wavelength of 248 nm, with 15-18 ns pulse width at half maximum, are reflected from the polymer surface placed at an . angle of 45° with respect to the laser beam. A portion of the central uniform region of the incident beam is focussed onto the target. The reflected light is detected with a fast photodiode and oscilloscope. The reflected pulse width at half maximum, reflected fluence, and reflected peak intensity are determined using a fresh surface for each laser pulse. Results are reported for polyimide, poly(ethylene terephthalate) (PET), polystyrene, poly(methyl methacrylate) (PMMA), and polyethylene, for incident fluences of 1-3000 mj/cm2.
© 1991 Optical Society of AmericaPDF Article