Picosecond coherent Raman studies of polymer laser microchemistry

Laser polymer microchemistry processes are widely used in photomicrolithography, electronics and many other high technology industries. Due to the complicated nature of the laser-material interactions and the diversity and complexity of the polymer thin-film materials, the fundamental mechanisms of these processes are poorly understood. Here we are interested in laser-assisted processes where an image is produced by chemically altering a micrometer sized spot of a thin film by a laser pulse. Some microchemistry processes of interest include laser ablation, photopolymerization, and surface modification. We have developed a new experimental technique which is generally applicable to thin films of any transparent polymer, which directly provides information about the temperature and pressure of a thin film and its chemical composition during the intense laser pulse which induces microchemistry. This technique is resistant to interference from background problems produced by the chemical reaction such as background fluorescence, and it can even be used on materials undergoing ablation, which involves a violent explosive reaction. In our experiments a 100 ps duration laser pulse is used to cause a chemical reaction in a micrometer sized spot of a polymer thin film cast onto a large-area glass substrate. Then a pair of 35 ps laser pulses incident on the same microvolume are used to generate the coherent Raman spectrum in a phase-mismatched multiplex CARS configuration. The mismatch serves to shorten the CARS coherence length, suppressing coherent background emission from the thicker glass substrate which supports the film, without significantly affecting the film signal. In initial experiments, we have studied laser ablation of a thin film of PMMA (polymethyl methacrylate), doped with a near-IR dye which absorbs near 1 μm. Near-IR pulses deposit into the film an amount of energy equivalent to an equilibrium temperature jump ranging from 200°C, which is enough to melt the film, to 600°C, which is enough to cause ablation. Coherent Raman spectra provided interesting information about the ultrafast temperature and pressure increases in these films, and indicate the partial depolymerization of the PMMA to methyl methacrylate on the 1-ns time scale. Picosecond CARS is a versatile and sensitive method of studying polymer laser microchemistry which is particularly attractive because it can generally be applied to polymer thin film systems of scientific and technological interest.