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.

© 1991 Optical Society of America

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