Microstructures formed in polypropylene by a single pulse of near-infrared ultrafast laser with nonlinear absorption

Polymeric materials are expected to interact with lasers differently depending on the presence of impurities in the material since the impurities can induce different absorption mechanisms. Especially ultrafast lasers of near -infrared wavelength may exhibit linear absorption at low intensities, but nonlinear absorption is found to occur at higher intensities. Two types of polypropylene samples, one containing impurity and the other relatively pure, are examined in this study for the nonlinearity in their absorption characteristics at a near-infrared wavelength. The absorptances of the samples are determined by measuring their optical properties, i.e., reflectance, trans-mittance, and the multiphoton excitation order is determined by considering a nonlinear intensity model for the propagation of the laser beam through the samples. In addition, the self-phase modulation of the laser is found to enhance the absorption mechanism by switching the absorption from the nonresonant to resonant mode. These variations in the absorption mechanisms can induce different damage features as observed in the polypropylene samples.