Ionization dynamics and damage conditions in fused silica irradiated with mid-infrared femtosecond pulses

The employment of ultrashort laser sources at the mid-infrared (mid-IR) spectral region for dielectrics is expected to open innovative routes for laser patterning and a wealth of exciting applications in optics and photonics. To elucidate the material response to irradiation with mid-IR laser sources, a consistent analysis of the interaction of long wavelength femtosecond pulses with dielectric materials is presented. The influence of the pulse duration is particularly emphasized in specifying the laser parameters for which photoionization and impact ionization are important. Simulation results using pulses at 2.2, 3.2, and 5 μm are conducted to illustrate optimum conditions for the onset of damage on the solid that is related to the occurrence of the optical breakdown. The results predict that the damage threshold scales as ∼τpa (0.31≤a≤0.37) at all laser wavelengths. Given the significant effect of the induced excitation level on excitation of surface plasmons (SPs), which account for the formation of laser-induced periodic structures oriented perpendicular to the laser polarization, a correlation of the produced electron densities with SPs and the threshold of SP excitation (∼τpβ, 0.33≤β≤0.39) are also discussed in this as yet unexplored spectral region. The results are expected to guide development of an innovative approach to surface patterning using strong mid-IR pulses for advanced applications.