ABLATION OF METAL-FILMS BY PICOSECOND LASER-PULSES IMAGED WITH HIGH-SPEED ELECTRON-MICROSCOPY

The ablation of free-standing metal films by picosecond laser pulses (50 ps, 0.6-8 J/cm2 532 nm) was visualized by electron microscopy. Triple-frame high-speed transmission electron microscopy was applied, with exposure times below 5 ns and frame repetition times greater than or similar to 20 ns. Ablation was observed to proceed by hole opening within 5-30 ns, speeding up with increasing laser fluence, and punching out of most of the melt. Once opened, the holes expanded by capillary forces with a velocity of almost-equal-to 100 m/s for some 10 ns. At large pulse fluences the evaporation was collimated along the film axis. This and the absence of patterned flow of the melt are conspicuous differences to the ablation of films by nanosecond laser pulses. The effects typical for picosecond pulses are attributed to an evaporation that is one-sided because of a thermal gradient along the film axis, and that exerts