Time-and space-resolved plasma absorption of a femtosecond laser pulse in dielectrics

During ultrafast laser ablation of dielectrics, the intense laser pulse ionizes the irradiated material and produces an optical breakdown region, or plasma, which is characterized by a high density of free electrons. These high-density electrons can efficiently absorb a large fraction of the laser irradiance energy, part of which will then be coupled into the bulk material, resulting in material removal through direct vaporization. The energy deposited into the material depends on the time- and space-dependent breakdown region, the plasma rise time, and the plasma absorption coefficient. Higher coupling efficiency results in higher material removal rate; thus energy deposition is one of the most important issues for ultrafast laser material processing. In the present work, a femtosecond breakdown model is applied to investigate energy deposition during ultrafast laser material interactions with water chosen as the particular material to investigate. One substantial contribution of the current work compared with the classical models is that the pulse propagation effect has been taken into account, which has been shown to become significant for pulse durations less than 10 picosecond. By accounting for the pulse propagation, the time- and space-resolved plasma evolution can be characterized with the femtosecond model, which, in turn, can determine the energy deposition through plasma absorption. With knowledge of the plasma absorption, changes in the pulse profile as it propagates in the focal region can be determined as well. Absorption of the laser pulse by plasma in water is compared to experimental data to validate the model. The present model also shows promise for determining the energy deposition in other transparent or moderately absorbing dielectric media during ultrafast laser processing.