Plasma Absorption of Femtosecond Laser Pulses in Dielectrics

Dielectric (high bandgap) materials represent an important and diverse class of materials in micro and nanotechnology, including MEMS devices, biomedical and bioengineering systems, multilayer thin film coatings, fiber optics, etc. Micromachining dielectrics using ultrafast lasers is an exciting and promising new research area with many significant advantages, including precision material removal, negligible heating of the workpiece, micron and sub-micron-size feature fabrication, and high aspect ratio features. During ultrafast laser processing of dielectrics, the intense laser pulse ionizes the irradiated material and produces an optical breakdown region, or plasma, that 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 materials processing, particularly for micron and sub-micron-scale laser materials processing. In the present work, a femtosecond breakdown model is developed to investigate energy deposition during ultrafast laser material interactions. One substantial contribution of the current work is that pulse propagation effects have been taken into account, which have been shown to become significant for pulse durations less than 10 ps. By accounting for the pulse propagation, the time and space-resolved plasma evolution can be characterized and used to 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 with experimental data to validate the model, as water is a well characterized dielectric. The model, however, is also applicable to other transparent or moderately absorbing solid and liquid dielectric media during ultrafast laser-materials interactions.