TIME-RESOLVED DYNAMICS OF SUBPICOSECOND LASER ABLATION

The ablation of lithium niobate (LiNbO3), POLY(tetrafluoroethylene) (PTFE, teflon), poly(methylmethacrylate) (PMMA) and polyimide (PI) by 500 fs UV excimer laser pulses at 248 nm is reported. Time-resolved measurements were carried out with pulse pairs of variable delay in the range from -200 to +200 ps. The ablation rate is very sensitive to the time delay between the two pulses, and -depending on the material and fluence-can increase or decrease for very short time delays. For LiNbO3, efficient shielding is observed within a few picoseconds. For PTFE and PMMA, and the total fluence just above threshold, the ablation rate versus time delay shows an autocorrelation type behavior with a full width at half-maximum below 400 fs, since two-photon absorption dominates the ablation process. For polyimide, excited state absorption is found to decrease the ablation rate for delay times below 30 ps.