Coherent Raman measurements of polymer thin-film pressure and temperature during picosecond laser ablation

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Abstract
Picosecond time‐resolved coherent Raman spectroscopy (ps CARS) is used to study photothermal ablation, induced by 150 ps duration near‐infrared optical pulses, of poly‐(methyl methacrylate) (PMMA) thin films doped with a small amount of near‐infrared absorbing dye. The pressure and temperature shifts of a PMMA transition at ≊808 cm−1 were calibrated in static P and T experiments. Dynamic frequency shifting of the PMMA transition is used to determine temperature and pressure in the ablating thin film, and to investigate the dynamics of fast thin‐film volume expansion. When the ablation pulse intensity is varied, ps CARS measurements of T and P are shown to be consistent with the results of conventional measurements below threshold, but near and above threshold picosecond time scale data show noticeable differences. Picosecond time scale ablation involves solid‐state shock waves, which are not produced by longer duration ablation pulses. A pressure jump, often several kbar, is produced when the film is heated faster than a characteristic hydrodynamic volume relaxation time τh. Pressure release occurs by shock rarefaction wave propagation. When the rarefaction wave reaches the substrate, a tensile force is exerted on the thin film, causing it to break away from the substrate. The pressure in the thin film at ablation threshold, Pabl≊0.5 GPa, is found to be generated by roughly equal contributions from shock and thermochemical polymer decomposition processes. Therefore the picosecond time scale ablation process is termed shock‐assisted photothermal ablation. The value of Pabl is interpreted to be the nanosecond time scale dynamic tensile strength of the thin film under conditions of ultrafast heating. It is found to be about one order of magnitude greater than the static strength of PMMA.