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A pressure gradient method for spectral broadening of intense-femtosecond laser pulses in gas-filled hollow fiber is proposed. The simulations using input energy of 6 mJ and pulse duration of 40 fs have shown that using the same value of ∫ p(x)dx, the energy transmittance can be enhanced by a factor of 25% compared to that of using constant gas pressure while the global spatial phase is also improved.

The nonlinear susceptibility of atoms interacting with an intense laser field has been investigated using numerical solution of the time-dependent Schrödinger equation. It was found that the behavior of nonlinear susceptibility strongly depends on the intensity; it increases linearly in low intensity regime (Kerr nonlinearity), then saturates at higher intensity. Furthermore, it has also been found that the nonlinear susceptibility is substantially influenced by the onset of ionization.

In this paper we treat by entirely ab initio methods, a number of phenomena in nonlinear dynamics in intense laser fields. In particular, we apply the recently developed Floquet-coupled-channel (FCC) method to investigate the phenomenon of adiabatic stabilization and formation of stabilization windows in hydrogen atom, as well as the frequency and intensity dependence of nonlinear-detachment process in hydrogen negative ion. Moreover, a solution is given, with examples, to the methodological problem of consistently evaluating the ATI-spectrum within the Floquet theory. Next, direct simulations are carried out to determine the influence of stabilization on the ATI-spectrum and on the bound state occupation probabilities of H-atom. Finally, the time-scale of formation of ATI-peaks is investigated and shown to be of the order of the rise-time of the pulse.