A three dimensional non-adiabatic model, recently developed [1], was used to study the propagation of femtosecond laser pulses in rare gases. Under high intensity field, generated by the short laser pulse, the gaseous medium ionizes and gives rise to a space-time dependence of the refractive index. In space domain, a Gaussian beam (or any peaked laser beam) produces higher electron concentration in its central part than in the outer region, which results in the defocusing of laser field. On the other hand, the refractive index modulation produced by an electron concentration variation in time induces the self-phase modulation of laser pulse. Since these two effects chiefly modify the pulse characteristics into unexpected one, control of ionization effects is an important issue in the investigation of the effects which the interaction between atoms and intense femtosecond laser pulses creates, such as high-order harmonics, self-channeling in air, and laser assisted particle acceleration. 
The spatial and temporal characteristics of the refractive index are investigated and corelated with the computed characteristics of the laser field which propagates in the gaseous plasma. The self-guided configuration, found in specific experimental conditions, are confirmed by our numerical simulations. 
The study of the pulse propagation proved to be essential in optimizing the high order harmonic generation process, both in space and time domains [2]. This was achieved by using self-guided pulses to optimize harmonic generation in space domain, and chirped pulses to optimize in time/frequency domain. The numerical results proved to be of basic importance in understanding the intricate processes of harmonic generation in the above conditions [3,4].

1. V. Tosa, E. Takahashi, Y. Nabekawa, K. Midorikawa, “Generation of High Order Harmonics in a Self-Guided Beam”, Phys. Rev. A 67, 063817 (2003)

2. H T Kim, I J Kim, D G Lee, K-H Hong, Y S Lee, V. Tosa, and C H Nam, "Optimization of high-order harmonic brightness in the space and time domains," Phys. Rev. A 69(R), 031805 (2004)

3. V. Tosa, H.T. Kim, C.H. Nam, “High order harmonic generation with chirped and self-guided laser pulses. II. Time-frequency analysis.”, Phys. Rev. A 71, 063808 (2005)

4. V. Tosa, H.T. Kim, I.J. Kim, C.H. Nam, “High order harmonic generation with chirped and self-guided laser pulses. I. Spatial and spectral analysis.”, Phys. Rev. A 71, 063807 (2005)