Coherent Raman Measurements of Polymer Thin-Film Pressure and Temperature DuringPicosecond Laser Ablation

摘要

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 approx. 808 per cm 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 pulse. (AN).