Ionization dynamics and damage conditions for transparent materials irradiated with Mid-Infrared femtosecond pulses

The employment of ultrashort laser sources at the mid-IR spectral region for transparent materials is designed to open new 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, in particularly, emphasized towards specifying the laser parameters for which photoionization and impact ionization are important. Simulations results using pulses at 2.2 mum, 3.2 mum and 5 mum are conducted to illustrate the optimum conditions for the onset of damage on the solid that is related to the occurrence of the optical breakdown. Results predict that the damage threshold scales as ~$\tau_p^\alpha$ (0.31<$\alpha$<0.37) at all laser wavelengths. Given the significant effect of the induced excitation level on the excitation of Surface Plasmons (SP) which account for the formation of laser induced periodic structures (LIPSS) oriented perpendicular to the laser polarization, a correlation of the produced electron densities with SP and the threshold of SP excitation (~$\tau_p^\beta$, 0.33<$\beta$<0.39) are also discussed in this yet unexplored spectral region. Results are expected to allow a novel surface patterning approach with strong mid-IR pulses for advanced applications.