Energy deposition of ultrashort laser pulses in polyvinyl chloride measured by two-color pump-probe shadowgraphy

We report on the energy deposition of ultrashort laser pulses with two different intensities of 6 TW cm - 2 and 143 TW cm - 2 in polyvinyl chloride, using two-color pump-probe shadowgraphy. The interaction is imaged during the pulse propagation into the medium and up to 200 μ s after. The shadowgraphs show that higher-intensity pulses induce significantly more ionization of the material. Furthermore, it is shown that lower-intensity pulses propagate over the entire visible range of the shadowgraph without major losses, while higher-intensity pulses fade significantly after about 200 μ s . After the interaction, pulses of both intensities produce a similar temperature-induced shadowgraphy pattern. The observations of the ionization process, the propagation of the pulse in the medium, and the thermalization are explained based on theory regarding ultrashort pulse laser interaction with dielectrics. Furthermore, the ablation craters produced by multiple consecutive pulses of both intensities are compared. The crater produced with lower-intensity pulses shows trails of material change significantly below the ablation crater, while these trails do not exist when using pulses with higher intensity. This is attributed to the differences in pulse propagation observed in the shadowgraphs of single pulses. This work demonstrates, for the first time, the intensity dependence of energy deposition of ultrashort laser pulses in polyvinyl chloride and its consequences for laser micromachining.