Pulse-to-pulse evolution of optical properties in ultrafast laser micro-processing of polymers

Polymers were one of the first materials to be processed by ultrafast lasers. However, the nature of absorption for near-infrared laser beams is not fully understood, and therefore it remains challenging to process polymeric materials with high energy efficiency. In this study, the pulse-to-pulse evolution of optical properties (reflectance, transmittance, and absorptance) of polypropylene (PP), which is an important polymeric material widely used in many industrial applications, is determined by performing time-resolved measurements for a wide range of pulse energies. The goal is to differentiate between linear and nonlinear absorption in different laser-matter interaction regimes and select the processing condition that yields the highest energy efficiency. The experiment is performed by recording the reflection and transmission of each laser pulse in an ellipsoidal mirror-based setup, which enables the collection of scattering reflection with nearly full coverage. Absorption is calculated from the experimental data, and a model consisting of linear and nonlinear absorption is used to analyze the results. It is found that PP undergoes a dramatic morphological change from pulse to pulse, which is accompanied by changes in optical properties, that is, the tuning of the laser condition to fully utilize the laser energy. Their results could help increase energy efficiency in ultrashort-pulsed laser processing of polymers toward the high-throughput operation.