Surface Modifications of PMMA Induced by Controlled Electronic and Molecular Vibrational Excitation applying Ultrafast Mid-IR Laser Radiation

Optimizing the processing of organic photovoltaic devices by laser radiation requires a fundamental understanding of the excitation processes during laser radiation–matter interaction. Spatially and temporally resolved pump–probe ellipsometry on poly(methyl methacrylate) (PMMA), excited by single-pulsed ultrafast mid-IR laser radiation, reveals electronic and vibrational excitation as competing excitation processes. Mid-IR laser radiation in the femtosecond regime induces mainly wavelength independent nonlinear excitation of electrons, as predicted by a theoretical model of the induced free charge carrier density. In contrast, mid-IR laser radiation in the picosecond regime enables linear vibrational excitation, provided the laser radiation frequency corresponds to a resonance frequency of PMMA. Thereby, a wavelength selective processing of organic materials may enable faster processing of multilayer systems depending on the polymer specific vibrational modes.