Femtosecond pulse compression in pressure-gas cells filled with argon

The nonlinear propagation of femtosecond pulses in pressure-gas cells filled with argon is investigated. By increasing the pressure for reaching peak power levels close to the threshold for self-focusing, it is shown that either group-velocity dispersion or multiphoton ionizing (MPI) sources can become key players for arresting the beam collapse. For input powers noticeably above critical, MPI rapidly dominates and the formation of self-guided filaments of light occurs. We discuss the dynamical role of MPI in shortening the pulse duration up to the optical cycle limit. Two different wavelength domains are commented. The influence of space-time focusing and self-steepening effects is furthermore discussed. Their respective roles in promoting shock structures are studied and shown to still promote pulse shortening in suitable power regimes. Finally, spectral broadening is analyzed and proven to be more important for large laser wavelengths. Numerical integration of the propagation equations is explained in the light of analytical arguments.